OS X: Escape path given to -isysroot flag

Ensure that paths containing spaces or other special characters are
escaped correctly on the command line.
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 'osx-sysroot-escape' into next

77b2e6f1 OS X: Escape path given to -isysroot flag
Help: Fix wording of CMP0049 and CMP0050 release notes

Also tweak description of policy CMP0049 to clarify that source
lists were not expanded, just variables in individual source names.
Merge topic 'backward-compatibility' into next

ddd0db99 Help: Fix wording of CMP0049 and CMP0050 release notes
Merge topic 'backward-compatibility' into next

f2eee72f add_custom_command: Disallow use of SOURCE signatures.
c248a437 Add policy CMP0049 to avoid variable expansion in source lists
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
Merge branch 'master' into next