Fix test compile error in long double broken environment.

This PR will fix following compile error.

[test_real_concept](http://www.boost.org/development/tests/develop/developer/output/FBSD-64%20jc-bell-boost-bin-v2-libs-math-test-test_real_concept-test-gcc-4-2-1-debug-link-static.html)
[test_constants](http://www.boost.org/development/tests/develop/developer/output/FBSD-64%20jc-bell-boost-bin-v2-libs-math-test-test_constants-test-gcc-4-2-1-debug-link-static.html)
[cstdfloat_concept_check_2](http://www.boost.org/development/tests/develop/developer/output/FBSD-64%20jc-bell-boost-bin-v2-libs-math-test-cstdfloat_concept_check_2-test-gcc-4-2-1-debug-link-static.html)