msvc: fix "REG_STARTEND" issue

In 897d68e7af (Makefile: use curl-config --cflags, 2020-03-26), we
taught the build process to use `curl-config --cflags` to make sure that
it can find cURL's headers.

In the MSVC build, this is completely bogus because we're running in a
Git for Windows SDK whose `curl-config` supports the _GCC_ build.

Let's just ignore each and every `-I<path>` option where `<path>` points
to GCC/Clang specific headers.

Reported by Jeff Hostetler in
https://github.com/microsoft/git/issues/275.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/vcbuild/scripts/clink.pl

@@ -23,7 +23,9 @@ while (@ARGV) {
 	    # before any "-l*" flags.
 	    $is_debug = 1;
 	}
-	if ("$arg" =~ /^-[DIMGOZ]/) {
+	if ("$arg" =~ /^-I\/mingw(32|64)/) {
+		# eat
+	} elsif ("$arg" =~ /^-[DIMGOZ]/) {
 		push(@cflags, $arg);
 	} elsif ("$arg" eq "-o") {
 		my $file_out = shift @ARGV;