mingw: enable stack smashing protector

To reduce Git for Windows' attack surface, we started using the Address
Space Layout Randomization and Data Execution Prevention features in
ce6a158 (mingw: enable DEP and ASLR, 2019-05-08).

To remove yet another attack vector, let's 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 on Windows: The latter appears to strike a better balance
between the performance impact and the provided safety.

In a non-scientific 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 git-for-windows#501

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

config.mak.uname

@@ -596,7 +596,8 @@ 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