MinGW: Use -static-libgcc when linking a DLL.

When commit 846e91e022 switched the linker
driver from dlltool/dllwrap to gcc, it became possible for linking to
choose shared libgcc.  Backends having loaded a module dynamically
linked to libgcc can exit abnormally, which the postmaster treats like a
crash.  Resume use of static libgcc exclusively, like 9.3 and earlier.
Back-patch to 9.4.

src/Makefile.shlib

@@ -371,16 +371,22 @@ else
 
 $(stlib): $(shlib) ;
 
+# XXX A backend that loads a module linked with libgcc_s_dw2-1.dll will exit
+# uncleanly, hence -static-libgcc.  (Last verified with MinGW-w64 compilers
+# from i686-4.9.1-release-win32-dwarf-rt_v3-rev1.)  Shared libgcc has better
+# support for C++/Java exceptions; while core PostgreSQL does not use them, it
+# would be nice to support shared libgcc for the benefit of extensions.
+#
 # If SHLIB_EXPORTS is set, the rules below will build a .def file from that.
 # Else we just use --export-all-symbols.
 ifeq (,$(SHLIB_EXPORTS))
 $(shlib): $(OBJS) | $(SHLIB_PREREQS)
-	$(CC) $(CFLAGS)  -shared -o $@  $(OBJS) $(LDFLAGS) $(LDFLAGS_SL) $(SHLIB_LINK) $(LIBS) -Wl,--export-all-symbols -Wl,--out-implib=$(stlib)
+	$(CC) $(CFLAGS)  -shared -static-libgcc -o $@  $(OBJS) $(LDFLAGS) $(LDFLAGS_SL) $(SHLIB_LINK) $(LIBS) -Wl,--export-all-symbols -Wl,--out-implib=$(stlib)
 else
 DLL_DEFFILE = lib$(NAME)dll.def
 
 $(shlib): $(OBJS) $(DLL_DEFFILE) | $(SHLIB_PREREQS)
-	$(CC) $(CFLAGS)  -shared -o $@  $(OBJS) $(DLL_DEFFILE) $(LDFLAGS) $(LDFLAGS_SL) $(SHLIB_LINK) $(LIBS) -Wl,--out-implib=$(stlib)
+	$(CC) $(CFLAGS)  -shared -static-libgcc -o $@  $(OBJS) $(DLL_DEFFILE) $(LDFLAGS) $(LDFLAGS_SL) $(SHLIB_LINK) $(LIBS) -Wl,--out-implib=$(stlib)
 endif
 
 endif # PORTNAME == cgywin

src/makefiles/Makefile.win32

@@ -76,4 +76,4 @@ win32ver.o: win32ver.rc
 
 # Rule for building a shared library from a single .o file
 %.dll: %.o $(WIN32RES)
-	$(CC) $(CFLAGS) -shared -o $@ $^ -Wl,--export-all-symbols $(LDFLAGS) $(LDFLAGS_SL) $(BE_DLLLIBS)
+	$(CC) $(CFLAGS) -shared -static-libgcc -o $@ $^ -Wl,--export-all-symbols $(LDFLAGS) $(LDFLAGS_SL) $(BE_DLLLIBS)