Fix error on __has_include with older clang (< 3.1)

Older clang (< 3.1) rejects __has_include(header), therefore use __has_include(&lt;header&gt;) instead.

see [Test output: teeks99-03f-Ubuntu12.04-64 - config - abi_test / clang-linux-3.0](http://www.boost.org/development/tests/develop/developer/output/teeks99-03f-Ubuntu12-04-64-boost-bin-v2-libs-config-test-abi_test-test-clang-linux-3-0-debug.html)