Do not pass compiler executable for AppleClang if it is the default

[jcar87]

While the compiler resides in /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc, and that is the path that CMake registers, this cannot be invoked directly unless ones passes flags to locate the sysroot with the library etc. CMake does pass this additional information, but other tools downstream might not (e.g. Autotools, as reported in an issue). This is otherwise done automatically when invoking via /usr/bin/cc, which in turn invokes it via xcrun, defining SDKROOT properly and thus it "just works".

The fix is to detect if the compiler CMake has detected is the default as found by xcrun - in that case, we don't propagate the compiler, and assume that it is the one that will be detected by default by downstream projects.

Close #589

[memsharded]

I'd say is looking good, better avoid the definition of compiler_executables.

[jcar87]

Additionally and come to think of it - when the CMake generator is "Visual Studio", we don't need to propagate compilers either - these are either derived by CMake itself, or Conan activates vcvars on the fly for command line generators. Less a problem, as it is redundant, but worth taking into account.