Switch to an external LibRaw

Instead of using an embedded copy of LibRaw, look for an external LibRaw
as mandatory dependency with a new CMake module and using its variables.

Considering some LibRaw versions seem to be underlinked and not linking
to OpenMP, link it manually in libkdcraw to overcome such lack.

Switch back to the MAKE_KDCRAW_LIB define (i.e. the default set by
KDE4_ADD_LIBRARY) as the one used to check whether it is being built,
as otherwise LIBRAW_BUILDLIB would conflict with LibRaw.

See review #110962