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. Fixes: debbugs:9755
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