bug#18302: MSYS2 build issues

[Karol Ostrovsky]

Dear Emacs maintainers,

thanks to the work of Chris Zheng and Eli Zaretskii we can build Emacs
on MSYS2 with MinGW-w64.  However, the process is not completely
painless: xpm support needs manual tweaking of CFLAGS, configure
script must be run with special flags, and make sometimes crashes for
the "emacs$(EXEEXT)" target.

I am submitting my proposed fixes for these three issues (based on the
latest trunk version of Emacs).  With this fix I can successfully
compile 64-bit Emacs using MSYS2 and MinGW-w64 with automatic xpm
support, and the process follows the same easy steps as described in
nt/INSTALL (just running ./autogen.sh, ./configure --prefix=..., and
make).  I intend to submit some additional MSYS2+MinGW-w64
documentation (file nt/INSTALL) should this patch be accepted.

Best regards,

Karol

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