Recently added ASTC support causing MinGW build issues

[Crunkle]

Whilst attempting to build bgfx using mingw-gcc-release64 and, as a result, the associated bimg_encode dependency, an error occurs due to the recent commit 03ad392 adding new third-party sources.

In file included from ../../../../bimg/3rdparty/astc/astc_codec_internals.h:22:0,
from ../../../../bimg/3rdparty/astc/astc_averages_and_directions.cpp:17:
../../../../bimg/3rdparty/astc/mathlib.h:30:7: warning: "_MSC_VER" is not defined [-Wundef]
#if (!_MSC_VER) && (__cplusplus < 201103L)
^~~~~~~~
In file included from /usr/lib/gcc/x86_64-w64-mingw32/6.3-win32/include/c++/cmath:45:0,
from /usr/lib/gcc/x86_64-w64-mingw32/6.3-win32/include/c++/math.h:36,
from ../../../../bimg/3rdparty/astc/astc_averages_and_directions.cpp:19:
/usr/share/mingw-w64/include/math.h: In function ‘int __isnanl(long double)’:
/usr/share/mingw-w64/include/math.h:573:65: error: lvalue required as left operand of assignment
xx = (int) (ld.ldt->lh.low | (ld.ldt->lh.high & 0x7fffffffu)); /* explicit */
^

Unsure whether this is a bug in MinGW or improper use of the headers - it is preventing the build either way and the only non-intrusive solution is to rollback to an older version.

[cloudwu]

I fixed this issue, see #17