This API converts a config.h.in file into config.h. This is useful when
introducing a build.zig file to an existing C/C++ project that is
configured with autotools or cmake.
The cmake syntax is not implemented yet.
This hack was initially introduced as we would export all symbols
unconditionally, including non-function definitions. This would cause
an error from the Wasmtime runtime engine, which this flag would
suppress. As we now properly export symbols, this flag is no longer
needed and any user running into this error can manually include it.
This commit also adds the `--import-symbols` ability to build.zig
