An assortment of GCC and CMake fixes and enhancements

[qyot27]

• The patch from pull request #9 has been updated to work with the current HEAD
• The convolution and histogram fixes are included.
• A good chunk are mostly intended to make it easier for MinGW-w64 cross-compiling, from case sensitivity to tidying up parts of the relevant CMakeLists.txt to facilitating better use of Ninja.
• Some warnings are now silenced.
• capi.h protected against the MSVC/GCC problem; defaults to MSVC-style mainstream use (resulting in 32-bit GCC builds not working with external programs like FFmpeg), but provides AVSC_WIN32_GCC32 define so projects like FFmpeg can switch to the alternate behavior.
• PluginDir+GCC registry entry added, so that GCC builds can be if-deffed to ignore plugins for MSVC builds, as well as have their own plugin directories. This way, GCC and MSVC builds can co-exist without hassling each other.