config.mak.uname: supporting 64-bit MSys2

This just makes things compile, the test suite needs extra tender loving
care in addition to this change. We will address these issues in later
commits.

While at it, also allow building MSys2 Git (i.e. a Git that uses MSys2's
POSIX emulation layer).
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -518,13 +518,12 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_INET_NTOP = YesPlease
 	NO_POSIX_GOODIES = UnfortunatelyYes
 	DEFAULT_HELP_FORMAT = html
-	COMPAT_CFLAGS += -D_USE_32BIT_TIME_T -DNOGDI -Icompat -Icompat/win32
+	COMPAT_CFLAGS += -DNOGDI -Icompat -Icompat/win32
 	COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"
 	COMPAT_OBJS += compat/mingw.o compat/winansi.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
 		compat/win32/dirent.o
 	BASIC_CFLAGS += -DPROTECT_NTFS_DEFAULT=1
-	BASIC_LDFLAGS += -Wl,--large-address-aware
 	EXTLIBS += -lws2_32
 	GITLIBS += git.res
 	PTHREAD_LIBS =
@@ -545,8 +544,17 @@ ifneq (,$(wildcard ../THIS_IS_MSYSGIT))
 else
 	ifeq ($(shell expr "$(uname_R)" : '2\.'),2)
 		# MSys2
+		prefix = /usr/
+		ifeq (MINGW32,$(MSYSTEM))
+			prefix = /mingw32
+		endif
+		ifeq (MINGW64,$(MSYSTEM))
+			prefix = /mingw64
+		else
+			COMPAT_CFLAGS += -D_USE_32BIT_TIME_T
+			BASIC_LDFLAGS += -Wl,--large-address-aware
+		endif
 		CC = gcc
-		prefix = /mingw32
 		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0
 		INSTALL = /bin/install
 		NO_R_TO_GCC_LINKER = YesPlease