Any time you wanted to know the list of gcc used macros and their values. Here is the command details
touch foo.h; gcc -dM -E foo.h
will list all the macros used.
-dM will generate the list of all #defines used by the pre-processor during execution, this also includes pre-defined macros.
-E will stop the compiler after pre-processing stage. Will not enter to the compiling stage. The output will be in the preprocessed form and will be sent to the stdout.