Enable C compiler warnings without setting CHPL_DEVELOPER.

Add new WARNINGS var in Makefiles. Set to 1 when CHPL_DEVELOPER is
defined. Use WARNINGS to enable warning flags in compiler specific makefiles.

Previously whether or not CHPL_DEVELOPER was defined determined if the warning
flags were added for C compiler. Instead, use the WARNINGS variable. This
allows a developers to build with compiler warnings enabled (e.g. when building
the runtime/) without the rest of the CHPL_DEVELOPER settings. Conversely, if
CHPL_DEVELOPER is set, a developer can disable C compiler warning flags by
setting WARNINGS=0 when calling make. The latter is not especially useful (or
recommended).

### TODO
* [x] Update docs regarding CHPL_DEVELOPER to include new side effect.
* ~~[ ] Full test pass.~~
* ~~[ ] Consider adding C compiler warning flags for PrgEnv C compilers.~~