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.
6 jobs for fix_gdal_ossfuzz_build in 12 minutes and 22 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #701845
build

00:12:22

passed #701850
build_android

00:01:06

passed #701846
build_clang_libcpp

00:03:39

passed #701849
build_clazy

00:07:01

passed #701848
build_mingw64_fedora

00:02:42

passed #701847
build_ubuntu_16_04

00:11:17