mingw: prepare for making git config --system do what users expect

This moves the system config into a more logical location: the `mingw64`
part of `C:\Program Files\Git\mingw64\etc\gitconfig` never made sense,
as it is a mere implementation detail. Let's skip the `mingw64` part and
move this to `C:\Program Files\Git\etc\gitconfig`.

Side note: in the rare (and not recommended) case a user chooses to
install 32-bit Git for Windows on a 64-bit system, the path will of
course be `C:\Program Files (x86)\Git\etc\gitconfig`.

The next step after this patch is to scrap support for
`C:\ProgramData\Git\config`, which never really made sense to users, and
which is inconsistent with non-Windows versions of Git, anyway.

Background: During the Git for Windows v1.x days, the system config was
located at `C:\Program Files (x86)\Git\etc\gitconfig`. With Git for
Windows v2.x, it moved to `C:\Program Files\Git\mingw64\gitconfig` (or
`C:\Program Files (x86)\Git\mingw32\gitconfig`).

Obviously, this new location was not stable (because of the "mingw64"
part) and this maintainer thought that it was a splendid time to
introduce a "super-system" config, with a constant location, that would
be shared by all Git implementations. Hence `C:\ProgramData\Git\config`
was born.

What we *should* have done instead is to move the location to the
top-level `etc` directory instead of the `mingw64\etc` one (or
`mingw32\etc` for 32-bit versions).

Likewise, we should have treated the system `gitattributes` in a similar
manner.

This patch makes it so.

Obviously, we are cautious to do this only for the known install
locations `/mingw64` and `/mingw32`; If anybody wants to override that
while building their version of Git (e.g. via `make prefix=$HOME`), we
leave the default location of the system config and gitattributes alone.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

config.mak.uname

@@ -425,6 +425,11 @@ ifeq ($(uname_S),Windows)
 	NATIVE_CRLF = YesPlease
 	INTERNAL_QSORT = YesPlease
 	DEFAULT_HELP_FORMAT = html
+ifeq (/mingw64,$(subst 32,64,$(prefix)))
+	# Move system config into top-level /etc/
+	ETC_GITCONFIG = ../etc/gitconfig
+	ETC_GITATTRIBUTES = ../etc/gitattributes
+endif
 
 	CC = compat/vcbuild/scripts/clink.pl
 	AR = compat/vcbuild/scripts/lib.pl
@@ -671,6 +676,11 @@ else
 		NO_CURL =
 		USE_NED_ALLOCATOR = YesPlease
 		NO_PYTHON =
+		ifeq (/mingw64,$(subst 32,64,$(prefix)))
+			# Move system config into top-level /etc/
+			ETC_GITCONFIG = ../etc/gitconfig
+			ETC_GITATTRIBUTES = ../etc/gitattributes
+		endif
 	else
 		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO
 		NO_CURL = YesPlease