32bit GCC 6.1 builds of x265 hangs at encoding

[Ma0]

If you use mingw-w64/GCC 6.1 toolchain compiled at SSE2 level (-march=pentium4 -mtune=generic) to build 32bit x265 (at -O3 optimize level), the output x265 encoder hangs at encoding (but 'x265 -V' works). The problem is '-march=i686' option unconditionally added by x265 for 32bit GCC build https://bitbucket.org/multicoreware/x265/src/a5362b9533f6a5b77740b4e8f97dba2555b6f929/source/CMakeLists.txt?at=default&fileviewer=file-view-default#CMakeLists.txt-184

In x264 code '-march=i686' is added only if there is no '-march' in CFLAGS http://git.videolan.org/?p=x264.git;a=blob;f=configure;h=7c01ebcba157c05aa1c0b05347bb9cd85001d4da;hb=HEAD#l664

We shouldn't add '-march=i686' for 32bit GCC if user set his own '-march' in CXXFLAGS.

[Deepthi Nandakumar]

cmake: apply -march=i686 for 32bit GCC only if there is no -march in CXXFLAGS (fixes #270)

→ <<cset bebae72f9db7>>