Provide DLLEXPORT markers for C functions via PG_FUNCTION_INFO_V1 macro.

This isn't really necessary for our own code, because we use a .DEF file
in MSVC builds (see gendef.pl), or --export-all-symbols in MinGW and
Cygwin builds, to ensure that all global symbols in loadable modules
will be exported on Windows.  However, third-party authors might use
different build processes that need this marker, and it's harmless
enough for our own builds.

To some extent, this is an oversight in commit e7128e8db, so back-patch
to 9.4 where that was added.

Laurenz Albe

Discussion: <A737B7A37273E048B164557ADEF4A58B539300BD@ntex2010a.host.magwien.gv.at>

src/include/fmgr.h

@@ -344,11 +344,17 @@ typedef const Pg_finfo_record *(*PGFInfoFunction) (void);
 
 /*
  *	Macro to build an info function associated with the given function name.
- *	Win32 loadable functions usually link with 'dlltool --export-all', but it
- *	doesn't hurt to add PGDLLIMPORT in case they don't.
+ *
+ *	As a convenience, also provide an "extern" declaration for the given
+ *	function name, so that writers of C functions need not write that too.
+ *
+ *	On Windows, the function and info function must be exported.  Our normal
+ *	build processes take care of that via .DEF files or --export-all-symbols.
+ *	We add PGDLLEXPORT nonetheless so that C functions built with a
+ *	different build process are guaranteed to be exported.
  */
 #define PG_FUNCTION_INFO_V1(funcname) \
-Datum funcname(PG_FUNCTION_ARGS); \
+extern PGDLLEXPORT Datum funcname(PG_FUNCTION_ARGS); \
 extern PGDLLEXPORT const Pg_finfo_record * CppConcat(pg_finfo_,funcname)(void); \
 const Pg_finfo_record * \
 CppConcat(pg_finfo_,funcname) (void) \