buildsystem, epj2make, ide: support Emscripten compiler.

buildsystem, epj2make, ide: support Emscripten compiler.
 - add ld and ar toolchain command options to compiler configuration.
 - add object and output file extension options to compiler config.
 - change windows style path seperator to linux style when exporting compiler config's environment variables.
 - add support for resolving compiler environment variables in compiler config's executable dirs.