Improve OMPT initialization code

Use of #ifdef OMPT_DEBUG was causing messages to be generated under normal
operation when the OpenMP library was compiled with KMP_DEBUG enabled.
Elsewhere, KMP_DEBUG evaluates assertions, but never produces messages during
normal operation. To avoid this inconsistency, set OMPT_DEBUG using a cmake
variable LIBOMP_OMPT_DEBUG.

While I was editing the associated ompt-specific.h and ompt-general.c files,
make the spacing and comments consistent.

Patch by John Mellor-Crummey

Differential Revision: http://reviews.llvm.org/D14355

llvm-svn: 252173