build: Clean up cflags for clang
build: Clean up cflags for clang

A few spurious warning flags found their way into your configure.ac. A
weird built-in change and a C++ specific one...

Also, tell clang to not print "ignored options" warnings to not warn
about -02 used with -g.

media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer