[WIP] Add llvm-installer clang builds on OS X targets

[es1024]

This change adds support for building thirdparty with llvm-installer clang on OS X targets.

The following changes were needed:

• -isystem /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX##.#.sdk added to preprocessor flags, to locate system headers
• def use_cppflags_env_var(): return True on several dependencies built with autotools, since the -isystem flag is needed for configure tests, and those tests often build with only CPPFLAGS, so we need the -isystem flag in CPPFLAGS and not CFLAGS/CXXFLAGS
• libunistring/gettext were moved to the COMMON group. These are only used for OS X thirdparty, and can be built earlier/without libc++ support (pure C libraries).
• Various is_linux() checks changed to is_llvm_installer_clang() checks
• Fixed bid build not using CFLAGS set by our build scripts for float128