Do not override user-defined CMAKE_C[XX]_FLAGS for clang
337585e3 unconditionnaly overrides any potential user-defined CMAKE_C[XX]_FLAGS. This for example breaks the GDAL oss-fuzz builds which build Poppler from source (see https://github.com/OSGeo/gdal/blob/master/gdal/fuzzers/build.sh#L54) So do the same as the GCC path where we save input CMAKE_C[XX]_FLAGS and reinject them in custom CMAKE_C{XX}_FLAGS_{build_configuration} The values are identical to GCC, execpt for the _DEBUG configuration where we remove '-O2 -fno-reorder-blocks -fno-schedule-insns -fno-inline' since clang does not support -fno-reorder-blocks and -fno-schedule-insns, so it is likely better to disable any optimization for proper debugging.