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 ce6a158561f9 (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 https://github.com/git-for-windows/git/issues/501 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>