Try to fix building with gcc >= 4.6 on Darwin.

Try to fix building with gcc >= 4.6 on Darwin.
Eg, hydra builds have been failing for some time because (?) of this.
In gcc < 4.6, unrecognized -no-fo options just cause a warning.
In 4.6 and later, they cause an error.

* configure.in: (CPP): Do not unconditionally set it on Darwin.
Instead, try to test if -no-cpp-precomp is accepted.
(CPP_TEST_OPTIONS, SPECIFIED_CPP): New.