Help: Format install() command documentation

Add inline markup and explicit markup blocks as appropriate.
cmInstallFilesGenerator: Add reference to calling cmMakefile

Add a Makefile member to the cmInstallFilesGenerator class and
populate it on construction.  This will be useful in a following
change to evaluate generator expressions with proper context.
install: Support generator expressions in FILES and PROGRAMS mode

Teach the install(FILES) and install(PROGRAMS) commands to evaluate
generator expressions in the list of files.

Extend the ExportImport test to cover installation cases involving
generator expressions.
Merge topic 'install-FILES-genex' into next

6e89c8a5 install: Support generator expressions in FILES and PROGRAMS mode
f11f7b34 cmInstallFilesGenerator: Add reference to calling cmMakefile
e190236c Help: Format install() command documentation