msvc: fix "REG_STARTEND" issue
In
897d68e7af82 (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>