Fix debug build.

Debug build is broken since some weeks due to a limitation of .a archieve format
regarding to files large than 4GB, this patch assumes we are always using GCC and
add -femit-struct-debug-baseonly to debug builds, according to GCC manual:

-femit-struct-debug-baseonly
    Emit debug information for struct-like types only when the base name of the
    compilation source file matches the base name of file in which the struct was defined.
    This option substantially reduces the size of debugging information, but at
    significant potential loss in type information to the debugger.

After this change, on x86-64 webcore_nix.a changed from >4GB to 1.4GB.

Reviewed by: Jesus Sanchez-Palencia