Run a "light" preprocessor for GCC

cpp -nostdinc program.c

Call cpp directly, e.g.

$ cat >foo.c <<EOF
#define FOO
#ifdef FOO
foo is defined
#else
foo is not defined
#endif
EOF

$ cpp foo.c
# 1 "foo.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "foo.c"


foo is defined

Of course, if you include any headers then those will be included in the output. One way to avoid that might be to just grep -v out the lines with #includes (or perhaps just ones with #include < and allow #include "). Or you could specify the -nostdinc option to remove just standard includes (but possibly leave in local libraries unless you specify include paths so that they won't be found) - this would warn about missing headers, though.

Edit: Or use the preprocessor itself to make the inclusion of headers conditional, wrap them in something like #ifndef TESTING_PREPROCESSOR and use -DTESTING_PREPROCESSOR.