def rrtmg_sw_gen_source(ext, build_dir):
thispath = config.local_path<EOL>module_src = []<EOL>for item in modules:<EOL><INDENT>fullname = join(thispath,'<STR_LIT>','<STR_LIT>','<STR_LIT>',item)<EOL>module_src.append(fullname)<EOL><DEDENT>for item in src:<EOL><INDENT>if item in mod_src:<EOL><INDENT>fullname = join(thispath,'<STR_LIT>',item)<EOL><DEDENT>else:<EOL><INDENT>fullname = join(thispath,'<STR_LIT>','<STR_LIT>','<STR_LIT:src>',item)<EOL><DEDENT>module_src.append(fullname)<EOL><DEDENT>sourcelist = [join(thispath,'<STR_LIT>'),<EOL>join(thispath,'<STR_LIT>')]<EOL>try:<EOL><INDENT>config.have_f90c()<EOL>return module_src + sourcelist<EOL><DEDENT>except:<EOL><INDENT>print('<STR_LIT>')<EOL>return None<EOL><DEDENT>
Add RRTMG_SW fortran source if Fortran 90 compiler available,
    if no compiler is found do not try to build the extension.