Where are the Predefined Macros defined?

[bx L]

Hello everyone,
     I notice that gcc has some predefined macros, which can be
obtained with " gcc -dM -E - < /dev/null". I guess many of them are
machine-specified, such as __LDBL_MANT_DIG__. However, I can't find
where these macros are defined when browsing GCC's source codes. I'm
very curious about how these macros are defined.
     Any ideas? Thanks.