CONFIG: improve config files for Darwin (#1667), BSD-csh syntax (#1668)

- use Clang instead of Gcc for Darwin since this is its system
  compiler. The user can force use of Gcc by using Gcc92 etc.

- make etc/cshrc sed check more robust.

- replace tcsh (${%var}) syntax with ("${var}" != "")

[Fixes and ideas from Alexey Matveichev]

CONFIG: do not source the gperftools environment by default

- this is now an used feature, but can be re-enabled by advanced users
  if required.

etc/config.sh/unset

@@ -137,6 +137,7 @@ unset FFTW_ARCH_PATH
 unset GPERFTOOLS_ARCH_PATH
 unset GMP_ARCH_PATH
 unset MPFR_ARCH_PATH
+unset LLVM_ARCH_PATH
 unset MESA_ARCH_PATH
 unset KAHIP_ARCH_PATH
 unset METIS_ARCH_PATH