In pkg-config, accept both "CFlags:" and "Cflags:"

In pkg-config, accept both "CFlags:" and "Cflags:"

matthieu@ noticed that "pkg-config libpkgconf --cflags" failed because
libpkgconf.pc has "CFlags:" with upper-case 'F', but we accepted only
"Cflags:" with lower-case 'f'.  freedesktop.org's pkg-config accepts
both "C[Ff]lags", but is otherwise sensitive to case.

Accept "CFlags" by mapping it to "Cflags" when reading the file.  Fail
if the file has more than one "C[Ff]lags".

ok millert@ matthieu@