Simple MCA vars for ext1, ext2, and pmix3 components to reflect what
the underlying PMIx library version is. For example:
```
$ ompi_info --param pmix pmix3x --parsable --level 9 | grep
library_version
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:value:PMIx library version 3.0.0 (embedded in Open MPI)
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:source:default
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:status:writeable
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:level:4
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:help:Version of the underlying PMIx library
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:deprecated:no
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:type:string
mca:pmix:pmix3x:param:pmix_pmix3x_library_version:disabled:false
```
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
