Revert "Put -Wno-error=unused-command-line-argument-hard-error-in-future in CFLAGS, CXXFLAGS, and CPPFLAGS for 10.8+ with clang-5.1+"

This reverts commit 9a6c39a2b8f4c47d6da8fd6cda1e5f23a72bd088.
This doesn't need also to go to the terminal.
Add flag to compiler lines to allow clang to build packages even with unused command arguments.
Add path-prefix-clang to PATH on 10.9.  path-prefix-libcxx is first in the PATH.
Since path-prefix-clang/ and path-prefix-libcxx/ are dynamically generated rather than being present in the .deb, delete them after fink is installed.  This allows new versions of the wrappers to be installed via Fink::PkgVersion::ensure_clang_prefix() and Fink::PkgVersion::ensure_libcxx_prefix(), respectively, during the next build operation.
Clang on 10.7 (Xcode 4.6 and presumably earlier) spews extra warnings about nonexistence of -Werror=unused-command-line-argument-hard-error-in-future, so add the flag only when supported by clang.
Merge pull request #109 from fink/refactor-compiler-wrapper

Refactor compiler wrapper