squash! mingw: use MSys2's libgen.h

mingw-w64-crt provides a basename() function, but it does not do what we
want. For example, it shortens `path0/` to `path`!

So let's not use it (i.e. let's revert commit 00bc8c9ea).

This fixes https://github.com/git-for-windows/git/issues/494

config.mak.uname

@@ -490,6 +490,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	HAVE_ALLOCA_H = YesPlease
 	NO_PREAD = YesPlease
 	NEEDS_CRYPTO_WITH_SSL = YesPlease
+	NO_LIBGEN_H = YesPlease
 	NO_POLL = YesPlease
 	NO_SYMLINK_HEAD = YesPlease
 	NO_UNIX_SOCKETS = YesPlease
@@ -540,7 +541,6 @@ ifneq (,$(wildcard ../THIS_IS_MSYSGIT))
 	HAVE_LIBCHARSET_H = YesPlease
 	NO_GETTEXT = YesPlease
 	COMPAT_CLFAGS += -D__USE_MINGW_ACCESS
-	NO_LIBGEN_H = YesPlease
 else
 	ifeq ($(shell expr "$(uname_R)" : '2\.'),2)
 		# MSys2
@@ -570,7 +570,6 @@ else
 	else
 		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO
 		NO_CURL = YesPlease
-		NO_LIBGEN_H = YesPlease
 	endif
 	OTHER_PROGRAMS += git-wrapper$(X)
 	BUILT_IN_WRAPPER = git-wrapper$(X)