[binutils-gdb/gdb-8.0-branch] Make libiberty/waitpid.c compile without warnings on MinGW

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 704f06fe081d9b17eb192c0db91d8367edcd6c9e ***

Author: Eli Zaretskii <eliz@gnu.org>
Branch: gdb-8.0-branch
Commit: 704f06fe081d9b17eb192c0db91d8367edcd6c9e

Make libiberty/waitpid.c compile without warnings on MinGW

libiberty/ChangeLog:

2017-05-31  Eli Zaretskii  <eliz@gnu.org>

	* waitpid.c (wait) [__MINGW32__]: Define as a macro
	that calls _cwait, so that this function works on MinGW.

(cherry picked from commit 90b4491842980d57a0e572167a168e04a476bca5)