llvm: build with GCC; no termcap; no bindings

By default LLVM/Clang attempts to build with Clang, not GCC. Clang
will be available on Darwin by default. Force it to use GCC.

If termcap library is on the system, it will attempt to use it.

Disable bindings otherwise depending on system packages such could
built (e.g., no need for OCaml bindings)

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
evtgen: sed must specify backup extension

On Unix system (e.g., Darwin) sed requires specifying backup file
extension for inplace modification. Set one to be compatible between
operating systems.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
pythia8: do not use -Qunused-arguments on GCC

By default Pythia8 assumes that on Darwin LLVM/Clang will be used and
-Qunused-arguments option is passed. Such option is not supported
by GCC. Modify configure script to check for Clang.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Merge pull request #1073 from davidlt/darwin-fix-cmssw-tool-conf

Resolve Darwin issues with cmssw-tool-conf