Fix for warnings from Intel compiler on macOS.#48

On macOS using the latest Intel compilers, the code emits these warnings: ```shell hedley.h(1261): warning #161: unrecognized #pragma HEDLEY_DIAGNOSTIC_PUSH ^ hedley.h(1287): warning #161: unrecognized #pragma HEDLEY_DIAGNOSTIC_POP ``` This seems to be because the Intel cxx compiler, icpc, defines the __clang__ macro. I added an additional check on the relevant part of the code to make sure an Intel compiler is not being used. I wasn't sure if there was a better way you prefer, but the warnings go away with this small change and did not seem to affect compilation with a clang compiler.