More generic version of subdir(if_found: list[dep])

[mitchgrout]

Often, you'll see code like the following within library code intended to be pulled in as a dependency:

if is_test
  subdir(<test directory>)
endif

...where is_test tends to be things like not meson.is_subproject() or get_option(...). There currently exists a kwarg on subdir() which allows for the subdir to be optionally visit if a list of dependencies has dep.found() to be all true, with the following equivalence:

subdir(target, if_found: deps)

# is equivalent to...
found = true
foreach dep: deps
  found = found and dep.found()
endforeach

if found
  subdir(target)
endif

My thought is to deprecate if_found in favour of a more potent argument which would support a wider array of argument types, including but not limited to:

1. dep --> Decides based off dep.found()
2. bool --> Decides based off value being true
3. feature --> Decides based off feature.enabled()

such that the following statements could all be written:

subdir('test', if: not meson.is_subproject()) 
subdir('test', if: get_option('enable-tests')) 
subdir('test', if: doctest.found()) 
subdir('test', if: doctest) 

There is some ambiguity as to how feature would be supported here, since there is both feature.enabled() and feature.allowed() which could both seemingly make sense here. Thoughts?

[mitchgrout]

Just a bit of a concrete example for motivation:

# Under the proposed enhancement:
is_test = not(meson.is_cross_build())
subdir('code')                           # Set up all of the code libraries
subdir('tests',        if:     is_test ) # Set up all the test executables
subdir('targets/exe',  if: not(is_test)) # Set up the final on-target executable
subdir('targets/test', if:     is_test ) # Attach our test executables via `test()`

# Without the proposed enhancement:
subdir('code')
if is_test
  subdir('tests')
endif

if not(is_test)
  subdir('targets/exe')
else
  subdir('targets/test')
endif

# Abusing if_found:
is_test  =     meson.is_cross_build()? disabler() : declare_dependency()
is_build = not meson.is_cross_build()? disabler() : declare_dependency() 

subdir('code')
subdir('tests',        if_found: is_test)
subdir('targets/exe',  if_found: is_build)
subdir('targets/test', if_found: is_test)