Escape individual COMPILE_OPTIONS entries.

Unlike the COMPILE_FLAGS contents, the COMPILE_OPTIONS is designed
to be escaped before being sent to the generator.
Add generator expressions for compiler versions.

New generator expressions allow retrieval of the version per language,
as well as equality comparison.
Revert "Add generator expressions for compiler versions."

This reverts commit 5d033a4c7362eb55819123111060d7b97ea67e9c.
Merge topic 'compiler-version-genex' into next

f87758f Revert "Add generator expressions for compiler versions."
Merge topic 'compiler-version-genex' into next

166bbb6 Add generator expressions for compiler versions.
b528f1c Escape individual COMPILE_OPTIONS entries.