Start supporting 64-bit MSys2

This just makes things compile, the test suite most likely needs extra
tender loving care in addition to this change.

While at it, also allow building MSys2 Git (i.e. a Git that uses MSys2's
POSIX emulation layer).

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

config.mak.uname

@@ -511,13 +511,12 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_POSIX_GOODIES = UnfortunatelyYes
 	DEFAULT_HELP_FORMAT = html
 	NO_D_INO_IN_DIRENT = YesPlease
-	COMPAT_CFLAGS += -D__USE_MINGW_ACCESS -D_USE_32BIT_TIME_T -DNOGDI -Icompat -Icompat/win32
+	COMPAT_CFLAGS += -D__USE_MINGW_ACCESS -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 =
@@ -537,9 +536,18 @@ ifneq (,$(wildcard ../THIS_IS_MSYSGIT))
 else
 	ifeq ($(shell expr "$(uname_R)" : '2\.'),2)
 		# MSys2
+		prefix = /usr/
+		ifeq (MINGW32,$(MSYSTEM))
+			prefix = /mingw32/
+		endif
+		ifeq (MINGW64,$(MSYSTEM))
+			prefix = /mingw64/
+		else
+			COMPAT_CFLAGS += -D_USE_32BIT_TIME_T
+			BASIC_LDFLAGS += -Wl,--large-address-aware
+		endif
 		CC = gcc
 		htmldir = share/doc/git/$(firstword $(subst -, ,$(GIT_VERSION)))/html
-		prefix = /mingw32/
 		INSTALL = /bin/install
 		NO_R_TO_GCC_LINKER = YesPlease
 		INTERNAL_QSORT = YesPlease