Add new emcc command line parameter --em-config that allows specifying the location of the .emscripten configuration file from the command line. This overrides and has the same meaning as the environment variable EM_CONFIG.
Add sanity test for --em-config emcc command line option.
Fix an issue with the --em-config cmdline option where the option was not seen by child processes spawned by emcc. To make sure all child processes see the same config file, the --em-config parameter is stored in the environment for current and child processes.
Merge branch 'em_config' of github.com:juj/emscripten into incoming

Conflicts:
	emcc