'0.0' isn't boolean false.
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.