graphics/rawtherapee: fix a few build glitches

In some circumstances, ${_MAKE_JOBS_NUMBER} remains undefined,
failing configure. Use ${MAKE_JOBS_NUMBER} instead.

Compile time optimizations:

• 5.7 regressed in that it would not use ccache. Reintroduce ccache where configured.
• Add -fno-fat-lto-objects to CFLAGS.

Cleanup duplicate LDFLAGS+=-Wl,--as-needed

PR: 240648