Autotools: Add pkg-config support for finding pcre2

To fix an old (and probably no longer manifest) build bug, this commit
adds the ability to detect an 8-bit libpcre2 with pkg-config in
addition to the other methods of specifying/detecting it. The
intention is that pkg-config will be used only in the default case,
where pcre support is detected automatically; that is, NOT in either
of these cases:

    * --with-pcre=<path> is given
    * --with-pcre=no is given

The code in pcre.m4 was modified minimally so that --with-pcre=yes
(the default) tries pkg-config first, and then falls back to whatever
it used to do. If pkg-config can find the library, we add PCRE_LIBS
to the LIBS variable and we're done. Otherwise, the old behavior is
retained.

ClamAV-bug: https://bugzilla.clamav.net/show_bug.cgi?id=12484
Gentoo-bug: https://bugs.gentoo.org/567680

This commit also collects the preprocessor flags obtained from
pkg-config.

When libpcre2 is found using pkg-config, we now say that explicitly,
and include its PCRE_LIBS and PCRE_CFLAGS in the output.

m4/reorganization/libs/pcre.m4

@@ -6,44 +6,81 @@ AC_ARG_WITH([pcre],[AS_HELP_STRING([--with-pcre@<:@=DIR@:>@],
     @<:@default=search PATH environment variable@:>@])],
   [pcreser=$withval],[pcreser="yes"])
 
-dnl determine if specified (or default) is valid
-case "$pcreser" in
-no)
-  pcreconfig=""
-  ;;
-yes)
-  dnl default - search PATH
-  AC_PATH_PROG([pcreconfig], [pcre2-config])
-  if test "x$pcreconfig" = "x"; then
-      AC_PATH_PROG([pcreconfig], [pcre-config])
-      if test "x$pcreconfig" = "x"; then
-          AC_MSG_NOTICE([cannot locate libpcre2 or libpcre within PATH])
-      else
-         pcrelib="pcre"
-      fi
-  else
+dnl Look for pcre-config or pcre2-config within the specified path,
+dnl or (by default) in the system's default search path. This is
+dnl the only place the value of --with-pcre is used.
+AS_CASE([$pcreser],
+  [no],
+  [pcreconfig=""],
+dnl
+  [yes],
+  [ dnl No path was specified, so we execute the default action, which is
+    dnl to search for PCRE on the system. First, we try pkg-config; if that
+    dnl doesn't work, we search for the pcre-config or pcre2-config programs
+    dnl in the system's search path. We look for the 8-bit library because
+    dnl that's what the fallback check did when pkg-config was introduced
+    dnl here. The name "PCRE" was chosen to match e.g. PCRE_CPPFLAGS from
+    dnl the non-pkgconfig branch.
+    PKG_CHECK_MODULES([PCRE], [libpcre2-8 >= 10.30], [
+      dnl We found libpcre2 with pkg-config. We leave $pcreconfig empty,
+      dnl so that the next big "if" branch below is skipped, and we
+      dnl therefore don't try to do anything further with pcre-config.
+      dnl The subsequent "if" block that tests $found_pcre is also
+      dnl skipped, leaving us at the very last conditional for $have_pcre
+      dnl and $pcrelib. We set those variables here so that HAVE_PCRE and
+      dnl USING_PCRE2 will be defined. Finally, we append the output of
+      dnl "pkg-config --libs" to the LIBS variable.
+      have_pcre="yes"
       pcrelib="pcre2"
-  fi
-  ;;
-"")
-  AC_MSG_ERROR([cannot assign blank value to --with-pcre])
-  ;;
-*)
-  AC_PATH_PROG([pcreconfig], [pcre2-config], [], [$pcreser/bin])
-  if test "x$pcreconfig" = "x"; then
-      AC_PATH_PROG([pcreconfig], [pcre-config], [], [$pcreser/bin])
+
+      # PCRE_LIBS contains the output of "pkg-config --libs" here,
+      # and likewise for PCRE_CFLAGS which is even more of a misnomer,
+      # as pkg-config --cflags outputs preprocessor flags.
+      LIBS="${LIBS} ${PCRE_LIBS}"
+      PCRE_CPPFLAGS="${PCRE_CPPFLAGS} ${PCRE_CFLAGS}"
+
+      dnl The summary at the end of ./configure checks that this is non-empty.
+      PCRE_HOME="pkg-config"
+      if test -n "${PCRE_LIBS}" || test -n "${PCRE_CFLAGS}"; then
+        PCRE_HOME="${PCRE_HOME} ( ${PCRE_LIBS} ${PCRE_CFLAGS} )"
+      fi
+    ], [
+      dnl We didn't find libpcre2 with pkg-config, fall back to pcre(2)-config.
+      AC_PATH_PROG([pcreconfig], [pcre2-config])
       if test "x$pcreconfig" = "x"; then
-          AC_MSG_ERROR([cannot locate libpcre2 or libpcre at $pcreser])
+          AC_PATH_PROG([pcreconfig], [pcre-config])
+          if test "x$pcreconfig" = "x"; then
+              AC_MSG_NOTICE([cannot locate libpcre2 or libpcre within PATH])
+          else
+             pcrelib="pcre"
+          fi
       else
-         pcrelib="pcre"
+          pcrelib="pcre2"
       fi
-  else
-      pcrelib="pcre2"
-  fi
-  ;;
-esac
+    ])
+  ],
+dnl
+  [""],
+  [AC_MSG_ERROR([cannot assign blank value to --with-pcre])],
+dnl default case:
+  [ AC_PATH_PROG([pcreconfig], [pcre2-config], [], [$pcreser/bin])
+    if test "x$pcreconfig" = "x"; then
+        AC_PATH_PROG([pcreconfig], [pcre-config], [], [$pcreser/bin])
+        if test "x$pcreconfig" = "x"; then
+            AC_MSG_ERROR([cannot locate libpcre2 or libpcre at $pcreser])
+        else
+           pcrelib="pcre"
+        fi
+    else
+        pcrelib="pcre2"
+    fi
+  ])
 
-dnl use pcre-config to check version, get cflags and libs
+dnl At this point we have either found pcre(2)-config, or not, and
+dnl the path to it is stored in $pcreconfig. If we found it, we use
+dnl it to get the PCRE version, CFLAGS, LIBS, et cetera. Note that
+dnl this next "if" will always fail if we found libpcre2 with pkg-
+dnl config.
 found_pcre="no"
 if test "x$pcreconfig" != "x"; then
     AC_MSG_CHECKING([pcre-config version])
@@ -101,7 +138,12 @@ if test "x$pcreconfig" != "x"; then
     AC_MSG_NOTICE([LIBS from pcre-config: $PCRE_LIBS])
 fi
 
-have_pcre="no"
+if test "x$have_pcre" != "xyes"; then
+    dnl default to "no" only if the pkg-config check hasn't already
+    dnl set it to "yes"
+    have_pcre="no"
+fi
+
 if test "x$found_pcre" != "xno"; then
   dnl save_LIBS="$LIBS"
   save_CPPFLAGS="$CPPFLAGS"