Link gcc helpers statically on MinGW (#488)

When compiling a 32bit dll with the mingw-w64 compiler, some 64bit
integer arithmetic operations are implemented using functions from
libgcc (e.g. __udivdi3 and __umoddi3 from libgcc_s_dw2-1.dll). This
unexpected dependency is inconvenient for applications.

The run-time dependency can be avoid by linking statically.

configure.ac

@@ -123,13 +123,15 @@ case $host in
 	os="windows"
 	threads="windows"
 	win_implementation="mingw"
+	LDFLAGS="${LDFLAGS} -static-libgcc"
 	;;
 *-msys*)
 	AC_MSG_RESULT([ (Windows back-end, using MSYS2)])
 	backend="windows"
 	os="windows"
 	threads="windows"
 	win_implementation="mingw"
+	LDFLAGS="${LDFLAGS} -static-libgcc"
 	;;
 *-cygwin*)
 	AC_MSG_RESULT([ (Windows back-end, using Cygwin)])