CMake has -DNDEBUG and -O3 as its default flags for Release builds.
Homebrew clears out the default CMake flags, which is fine for
optimization because Homebrew passes its own optimization flag(s).
-DNDEBUG wasn't added back in, though.
This ensures -DNDEBUG is passed to CMake release builds by default,
instead of individual formulas having to add it explicitly.
This also removes explicit additions of -DNDEBUG from the formulae that
had them -- gflags, llvm, and taglib.
ClosesHomebrew/homebrew#47378.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>