multimedia/mpv configure fails on netbsd-9/i386

["John D. Baker" <jdbaker%consolidated.net@localhost>]

Attempting to build "multimedia/mpv" on netbsd-9/i386 fails during the
configure stage with:

[...]
Checking for C11 stdatomic.h                                              : no 
C11 atomics are required; you may need a newer compiler
*** Error code 1

Stop.
make[1]: stopped in /x/pkgsrc/multimedia/mpv
*** Error code 1

Stop.
make: stopped in /x/pkgsrc/multimedia/mpv


ISTR some discussion of this in which it was suggested to add "-march=i586"
to CFLAGS/CXXFLAGS.  Although the configure script appears to be testing for
the existence of a particular header file, this seems to work (adding:

.if "${PKGPATH}" == "multimedia/mpv" 
  CFLAGS+=      -march=i586
  CXXFLAGS+=    -march=i586
.endif 

to "/etc/mk.conf").

Perhaps an appropriate platform/architecture-specific conditional should
be added to "multimedia/mpv/Makefile"?

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