- 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.