headerscheck: Use ICU_CFLAGS

Otherwise, headerscheck will fail if the ICU headers are in a location
not reached by the normal CFLAGS/CPPFLAGS:

../src/include/utils/pg_locale.h:21:10: fatal error: unicode/ucol.h: No such file or directory

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/1127775.1754417387%40sss.pgh.pa.us

src/tools/pginclude/headerscheck

@@ -44,6 +44,7 @@ CXXFLAGS=${CXXFLAGS:- -fsyntax-only -Wall}
 MGLOB="$builddir/src/Makefile.global"
 CPPFLAGS=`sed -n 's/^CPPFLAGS[ 	]*=[ 	]*//p' "$MGLOB"`
 CFLAGS=`sed -n 's/^CFLAGS[ 	]*=[ 	]*//p' "$MGLOB"`
+ICU_CFLAGS=`sed -n 's/^ICU_CFLAGS[ 	]*=[ 	]*//p' "$MGLOB"`
 CC=`sed -n 's/^CC[ 	]*=[ 	]*//p' "$MGLOB"`
 CXX=`sed -n 's/^CXX[ 	]*=[ 	]*//p' "$MGLOB"`
 PG_SYSROOT=`sed -n 's/^PG_SYSROOT[ 	]*=[ 	]*//p' "$MGLOB"`
@@ -64,11 +65,11 @@ if $cplusplus; then
 	    -I*|-D*) CXXPPFLAGS="$CXXPPFLAGS $flag";;
 	  esac
 	done
-	COMPILER_FLAGS="$CXXPPFLAGS $CXXFLAGS"
+	COMPILER_FLAGS="$CXXPPFLAGS $CXXFLAGS $ICU_CFLAGS"
 else
 	ext=c
 	COMPILER=${CC:-gcc}
-	COMPILER_FLAGS="$CPPFLAGS $CFLAGS"
+	COMPILER_FLAGS="$CPPFLAGS $CFLAGS $ICU_CFLAGS"
 fi
 
 # Create temp directory.