Help: Revise and format add_test() command documentation

Favor the add_test(NAME) signature and document the limitations of
the plain signature.
ExternalProject: Add option to always run the build step

Teach ExternalProject_Add a new BUILD_ALWAYS option to skip using
the build step stamp file and execute the step on every build.

Extend the BuildDepends test with a case to cover this option.
tests: Add variable expansion tests

There are some corner cases in variable expansion which would be nice to
capture before going and rewriting the variable expansion code. The
majority of these are related to configuring files and strings with '@'
in them in conjunction with @ONLY being specified. Another is testing
for '(' usage inside of ENV variable references based on whether it is
quoted or not.
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.
Always set version variable of find_dependency macro

If there is no ARGV1, that is fine; version will be made empty, and no
version will be passed to find_package().

This is relevant when find_dependency is invoked multiple times,
sometimes with a version specified and sometimes without.

 find_dependency(dep1 3.4)
 find_dependency(dep2) # version still set to 3.4.
Merge topic 'doc-add_test-cleanup'

8afa5fe8 Help: Revise and format add_test() command documentation
Merge topic 'install-FILES-genex'

6e89c8a5 install: Support generator expressions in FILES and PROGRAMS mode
f11f7b34 cmInstallFilesGenerator: Add reference to calling cmMakefile
e190236c Help: Format install() command documentation
Merge topic 'fix-find_dependency-macro'

5878c5d9 Always set version variable of find_dependency macro
Merge topic 'ExternalProject-BUILD_ALWAYS'

73e5c6ae ExternalProject: Add option to always run the build step
Merge topic 'variable-expansion-tests'

a9bdef2d tests: Add variable expansion tests