mingw: enable stack smashing protector

As suggested privately to Brendan Forster by some unnamed person
(suggestion for the future: use the public mailing list, or even the
public GitHub issue tracker, that is a much better place to offer such
suggestions), we should make use of gcc's stack smashing protector that
helps detect stack buffer overruns early.

Rather than using -fstack-protector, we use -fstack-protector-strong
because it strikes a better balance between how much code is affected
and the performance impact.

In a local test (time git log --grep=is -p), best of 5 timings went from
23.009s to 22.997s (i.e. the performance impact was *well* lost in the
noise).

This fixes https://github.com/git-for-windows/git/issues/501

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

config.mak.uname

@@ -631,7 +631,8 @@ else
 			BASIC_LDFLAGS += -Wl,--large-address-aware
 		endif
 		CC = gcc
-		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -DDETECT_MSYS_TTY
+		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -DDETECT_MSYS_TTY \
+			-fstack-protector-strong
 		EXTLIBS += -lntdll
 		INSTALL = /bin/install
 		NO_R_TO_GCC_LINKER = YesPlease