Win: Use YASM if it is in the PATH and NASM isn't Previously, simd/CMakeLists.txt was hard-coded to use NASM, and it was necessary to override the NASM variable in order to use YASM. This commit changes the behavior such that NASM is still preferred, but YASM will be used if it is in the PATH and NASM isn't available. This brings the actual behavior in line with the behavior described in BUILDING.md. Based on https://github.com/xpol/libjpeg-turbo/commit/b0799a1598782799d4876538eddca7ad8438d8a6 Closes #107