Simplify find_clang.cr

[docelic]

There is a bit of overlap between clang/find_clang.cr and clang/CMakeLists.txt.

find_clang supports finding the clang++ binary, but this is redundant as CMake will figure it out and call find_clang with option --clang PATH.

Also, as @kalinon mentions in #32 , probably more optimizations can be made by directly reading values from one of:

llvm-config
    --cxxflags
    --ldflags
    --libs all
    --includedir

Instead of parsing those values out manually from parts of clang++ output.

[kalinon]

llvm-config --binpath as well