config.mak.uname: support MSys2

Git for Windows lags a little bit behind with the 2.x releases because
the Git for Windows developers wanted to let that big jump coincide with
a well-needed overhaul of the context within which Git for Windows is
developed.

To understand why this is such a big issue, it needs to be noted that
many parts of Git are not written in portable C, but instead relies on a
POSIX shell and Perl to be available. Even in the portable C part, there
is the ingrained notion that we can work with UTF-8 encoded strings.

To support the scripts, Git for Windows has to ship a minimal POSIX
emulation layer with Bash and Perl thrown in, and when the Git for
Windows effort started originally, in August 2007, we settled on using
MSys, a stripped down version of Cygwin. Consequently, the original name
of the project was "msysGit" (which, sadly, caused a *lot* of confusion
because few Windows users know about MSys, and even less care).

To compile the C code of Git for Windows, we used MSys, too: it sports
an additional version of the GNU C Compiler that targets the plain
Win32 API (with a few convenience functions thrown in) instead of the
POSIX emulation layer that would require the MSys runtime to run the
compiled programs. That way, Git for Windows' executable(s) are really
just Win32 programs. To discern executables requiring the POSIX
emulation layer from the ones that do not, the latter are called MinGW
(Minimal GNU for Windows) when the former are called MSys executables.

This reliance on MSys incurred challenges, too, though: some of our
changes to the MSys runtime -- necessary to support Git for Windows
better -- were not accepted upstream, the MSys runtime was not developed
further to support e.g. UTF-8 or 64-bit, and apart from not having a
package management system until much later (when mingw-get was
introduced), many packages provided by the MSys/MinGW project lag behind
the respective source code versions, in particular Bash and OpenSSL. For
a while, the Git for Windows project tried to remedy the situation by
trying to build newer versions of those packages, but the situation
quickly became untenable, especially with problems like the Heartbleed
bug requiring swift action and Git for Windows contributors being scarce
-- despite millions of downloads suggesting that there are many users.

After a brief push in the direction of mingw-get, thanks to the
long-time contributor and co-maintainer Sebastian Schuberth, it became
clear that we need to look for alternatives.

Happily, in the meantime the MSys2 project (https://msys2.github.io/)
emerged, and was chosen to be the base of the Git for Windows 2.x. MSys2
is a rewrite of the spirit of MSys: it is again a stripped down version
of Cygwin, but it is actively kept up-to-date with Cygwin's source code.
Thereby, it already supports Unicode internally, and it also offers the
64-bit support that we yearned for since the beginning of the Git for
Windows project.

MSys2 also ported the Pacman package management system from Arch Linux
and uses it heavily. This brings the same convenience to which Linux
users are used to from `yum` or `apt-get`, and to which MacOSX users are
used to from Homebrew or MacPorts, or BSD users from the Ports system,
to MSys2: a simple `pacman -Syu` will update all installed packages to
the newest versions currently available.

MSys2 is also *very* active, typically providing package updates
multiple times per week.

It still required a two-month effort to bring everything to a state
where Git's test suite passes, and a couple of patches await their
submission to the respective upstream projects. Yet without MSys2, the
modernization of Git for Windows would simply not have happened.

This commit lays the ground work to supporting MSys2-based Git builds.

Assisted-by: Waldek Maleska <weakcamel@users.github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

squash! config.mak.uname: support MSys2

Make sure that the nedmalloc patch is applied before.

config.mak.uname

@@ -520,7 +520,7 @@ 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_32BIT_TIME_T -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 \
@@ -543,9 +543,25 @@ ifneq (,$(wildcard ../THIS_IS_MSYSGIT))
 	INTERNAL_QSORT = YesPlease
 	HAVE_LIBCHARSET_H = YesPlease
 	NO_GETTEXT = YesPlease
-	USE_LIBPCRE= YesPlease
+	COMPAT_CLFAGS += -D__USE_MINGW_ACCESS
 else
-	NO_CURL = YesPlease
+	ifeq ($(shell expr "$(uname_R)" : '2\.'),2)
+		# MSys2
+		CC = gcc
+		prefix = /mingw32
+		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0
+		INSTALL = /bin/install
+		NO_R_TO_GCC_LINKER = YesPlease
+		INTERNAL_QSORT = YesPlease
+		HAVE_LIBCHARSET_H = YesPlease
+		NO_GETTEXT = YesPlease
+		USE_LIBPCRE= YesPlease
+		NO_CURL =
+		USE_NED_ALLOCATOR = YesPlease
+	else
+		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO
+		NO_CURL = YesPlease
+	endif
 endif
 endif
 ifeq ($(uname_S),QNX)