Mock up Xcode devdir to make Qt5 work on Mac OS X (#1832)

* build_environment: allow compilers to set up an environment

* clang: mock up a toolchain directory for xcode

Some projects ignore CC and CXX flags and instead use xcode to find the
toolchain. Clang on Apple should set up the environment properly.
Arguably, every compiler could do this on Apple, but let's see how this
works out just for AppleClang for now.

The Documentation directory is ~1.7G and the excluded platforms add up
to about 7G. Ignoring swift saves another 500M. The resulting Xcode.app
copy is in the 2G range.

* compiler: set member variables early

This is required so that later methods can query things such as the
version of the compiler.

* compiler: support finding the real path of the compiler

On Apple, the /usr/bin compilers are actually wrapping tools themselves
which query xcrun for the currently selected Xcode installation. Pierce
this veil and get the real, full path the to underlying compilers
instead.

* icu4c: install with rpath

On macOS, icu installs with a library ID of the library name. Enabling
rpath makes its ID its full installed path which lets Qt5 link against
it successfully.

* qt: no -no-gtkstyle flag on Qt5 on macOS