Windows support: Use pg_config_os.h to include port/win32.h if needed

We previously had the hardcoded macOS ("darwin") contents for this file,
which is unnecessary and incorrect. Instead keep this file empty in all
cases except on Windows, where we include port/win32.h (just like Postgres
itself does), since it is actually necessary for our use case.

Different from Postgres itself we undef PGDLLIMPORT and PGDLLEXPORT, since
they are not needed for pg_query, and PGDLLIMPORT conflicts with the
thread local attribute.

Additionally, we workaround a Windows-specific problem with sigsetjmp
defines when using clang whilst __MINGW64__ is defined (which can happen
on MSYS2-based environments).

Makefile

@@ -151,6 +151,17 @@ extract_source: $(PGDIR)
 	mkdir ./src/postgres
 	mkdir ./src/postgres/include
 	LIBCLANG=/Library/Developer/CommandLineTools/usr/lib/libclang.dylib ruby ./scripts/extract_source.rb $(PGDIR)/ ./src/postgres/
+	# Override OS-specific pg_config_os.h to only load Win32 logic (the primary port logic that matters for libpg_query), if needed
+	echo "#if defined(_WIN32) || defined(_WIN64)" > ./src/postgres/include/pg_config_os.h
+	echo "#include \"port/win32.h\"" >> ./src/postgres/include/pg_config_os.h
+	# Don't mark anything as visible based on how Postgres defines it
+	echo "#undef PGDLLIMPORT" >> ./src/postgres/include/pg_config_os.h
+	echo "#undef PGDLLEXPORT" >> ./src/postgres/include/pg_config_os.h
+	# Avoid getting incorrect sigsetjmp overrides
+	echo "#ifdef __clang__" >> ./src/postgres/include/pg_config_os.h
+	echo "#undef __MINGW64__" >> ./src/postgres/include/pg_config_os.h
+	echo "#endif /* __clang__ */" >> ./src/postgres/include/pg_config_os.h
+	echo "#endif" >> ./src/postgres/include/pg_config_os.h
 	# Adjust version string to ignore differences in build environments
 	sed -i "" '$(shell echo 's/\#define PG_VERSION_STR .*/#define PG_VERSION_STR "PostgreSQL $(PG_VERSION) \(libpg_query\)"/')' ./src/postgres/include/pg_config.h
 	# Copy version information so its easily accessible

src/postgres/include/pg_config_os.h

@@ -1,8 +1,8 @@
-/* src/include/port/darwin.h */
-
-#define __darwin__	1
-
-#if HAVE_DECL_F_FULLFSYNC		/* not present before macOS 10.3 */
-#define HAVE_FSYNC_WRITETHROUGH
-
+#if defined(_WIN32) || defined(_WIN64)
+#include "port/win32.h"
+#undef PGDLLIMPORT
+#undef PGDLLEXPORT
+#ifdef __clang__
+#undef __MINGW64__
+#endif /* __clang__ */
 #endif