Add "-fpermissive" flag for MinGW-w64 to suppress casting errors that prevent compilation

Compiling with 64-bit MinGW throws errors about losing precision when casting between different sized pointers. These errors prevent the build process from completing. This is a temporary fix until the long term goal of removing all compiler errors is met.
Merge pull request #382 from tpaviot/review/mingw-w64-flags

Add "-fpermissive" flag for MinGW-w64 to suppress casting errors that pr...