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/paraview

@@ -64,7 +64,7 @@ pv_api=auto             # Either auto or pair of (major.minor) digits
 # Compiler-specific location for ThirdParty installations
 archDir="$WM_THIRD_PARTY_DIR/platforms/$WM_ARCH$WM_COMPILER"
 
-# Clean PATH and LD_LIBRARY_PATH
+# Clean path and library path
 eval \
     "$($WM_PROJECT_DIR/bin/foamCleanPath -sh-env=PATH \
     $ParaView_DIR $archDir/ParaView- $archDir/qt- $archDir/cmake-)"