mingw: only enable the MSYS2-specific stuff when compiling in MSYS2

The tell-tale is the presence of the `MSYSTEM` value while compiling, of
course. In that case, we want to ensure that `MSYSTEM` is set when
running `git.exe`, and also enable the magic MSYS2 tty detection.

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -758,12 +758,12 @@ ifeq ($(uname_S),MINGW)
 		prefix = $(MINGW_PREFIX)
 		HOST_CPU = $(patsubst %-w64-mingw32,%,$(MINGW_CHOST))
 		BASIC_LDFLAGS += -Wl,--pic-executable
+		COMPAT_CFLAGS += -DDETECT_MSYS_TTY
                 ifeq (MINGW32,$(MSYSTEM))
 			BASIC_LDFLAGS += -Wl,--large-address-aware
                 endif
         endif
-	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -DDETECT_MSYS_TTY \
-		-fstack-protector-strong
+	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -fstack-protector-strong
 	EXTLIBS += -lntdll
 	EXTRA_PROGRAMS += headless-git$X
 	INSTALL = /bin/install