FindBoost: Search next to MPI libraries for boost_mpi (#14739)

Some distributions place boost_mpi next to the MPI libraries against
which it was built instead of next to the other Boost libraries.  If
find_package(MPI) has already been run prior to find_package(Boost) then
MPI_CXX_LIBRARIES or MPI_C_LIBRARIES may be set to the location of the
MPI libraries.  Teach FindBoost.cmake to look there for boost_mpi and
boost_mpi_python after looking next to the other Boost libraries but
not consider the location to be Boost_LIBRARY_DIR.
OS X: Split Intel compiler information files

Create platform information modules Platform/Darwin-Intel-(C|CXX).cmake
and helper module Platform/Darwin-Intel.cmake.  Teach existing module
Platform/Darwin-Intel-Fortran.cmake to use the helper too.  Move
information from Platform/Darwin-icc.cmake into these files and drop
information already in Platform/Darwin.cmake to avoid duplication.
Export: Use the CMAKE_DEVEL_VERSION macro for build-export files.

Move the macro definition to the cmExportBuildFileGenerator.h header
to share it.
Sublime: Pass language and config properly
Add policy CMP0049 to avoid variable expansion in source lists
add_custom_command: Disallow use of SOURCE signatures.

Add CMP0050 to control this behavior.
Merge topic 'FindBoost-MPI-hints'

a22eeca3 FindBoost: Search next to MPI libraries for boost_mpi (#14739)
Merge topic 'osx-intel-info-split'

3693c63f OS X: Split Intel compiler information files
Merge topic 'cmake-devel-version-macro'

6ca6613a Export: Use the CMAKE_DEVEL_VERSION macro for build-export files.
Merge topic 'dev/fix-sublime-compile-flags'

2d9f93ab Sublime: Pass language and config properly
Merge topic 'backward-compatibility'

f2eee72f add_custom_command: Disallow use of SOURCE signatures.
c248a437 Add policy CMP0049 to avoid variable expansion in source lists