provide a way for config tests to supply conditional compiler and linker arguments

For certain features in webassembly - namely pthreads/thread feature, the pthread libraries only become available when passing

'-s USE_PTHREADS=1' to the linker, otherwise the config test will fail when the user requests building the threaded version of webassembly. (threads are off by default)

We need a way to add linker flags to the config test for a feature when the user requests a certain feature.