meson: fix openssl detection issues in 6a30027

When not detecting openssl via pkg-config, we'd error out if the headers
weren't found, even if -Dssl=auto. When detecting via pkg-config, but the
headers could not be found, we'd error out because the ssl_int variable would
not exist.

Reported-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/20230313180432.GA246741@nathanxps13

meson.build

@@ -1189,23 +1189,29 @@ if sslopt in ['auto', 'openssl']
 
   # via pkg-config et al
   ssl = dependency('openssl', required: false)
+  # only meson >= 0.57 supports declare_dependency() in cc.has_function(), so
+  # we pass cc.find_library() results if necessary
+  ssl_int = []
 
   # via library + headers
   if not ssl.found()
     ssl_lib = cc.find_library('ssl',
       dirs: test_lib_d,
       header_include_directories: postgres_inc,
-      has_headers: ['openssl/ssl.h', 'openssl/err.h'])
+      has_headers: ['openssl/ssl.h', 'openssl/err.h'],
+      required: openssl_required)
     crypto_lib = cc.find_library('crypto',
       dirs: test_lib_d,
-      header_include_directories: postgres_inc)
+      required: openssl_required)
-    ssl_int = [ssl_lib, crypto_lib]
+    if ssl_lib.found() and crypto_lib.found()
-
+      ssl_int = [ssl_lib, crypto_lib]
-    ssl = declare_dependency(dependencies: ssl_int,
+      ssl = declare_dependency(dependencies: ssl_int, include_directories: postgres_inc)
-                             include_directories: postgres_inc)
+    endif
   elif cc.has_header('openssl/ssl.h', args: test_c_args, dependencies: ssl, required: openssl_required) and \
        cc.has_header('openssl/err.h', args: test_c_args, dependencies: ssl, required: openssl_required)
     ssl_int = [ssl]
+  else
+    ssl = not_found_dep
   endif
 
   if ssl.found()