cmake: Enable features based on the result of multiple find_package() calls - build

Question

I have a project that contains a visual debugger that I want to build only if a group of packages were found. I don't want to force a user that just want to build the base application to install all the debugger dependencies, so I don't mark then as REQUIRED when running find_package().
Right now my build looks like this:
./CMakeList
find_package(gazebo)
find_package(OpenGL)
find_package(GLUT)
add_subdirectory(debugger)
./debugger/CMakeList
if(NOT gazebo_FOUND OR NOT OpenGL_FOUND OR NOT GLUT_FOUND)
return()
endif()
This works just fine, but as the list of dependencies for the debugger grows, using this if to enable/disable the debugger build looks clumsy.
Is there a better alternative to enable the build of this sub-directory base on the result of find_package()?
The debugger is not the only thing in my project that I enable/disable this way, so I need a generic solution for enabling features based on the packages found.

Answer 1

CMake has no already-made a macro or a function to check list of variables at once.
But it is not a difficult to write a function which do that. E.g.:
# Usage: feature_check(FEATURE_VAR REQUIRED_VAR [REQUIRED_VAR2 ...])
#
# Set FEATURE_VAR to TRUE if all required vars are TRUE,
# Otherwise set FEATURE_VAR to FALSE.
function(feature_check FEATURE_VAR REQUIRED_VAR)
foreach(var ${REQUIRED_VAR} ${ARGN})
if(NOT ${var})
set(${FEATURE_VAR} "FALSE" PARENT_SCOPE)
return()
endif(NOT ${var})
endforeach(var ${REQUIRED_VAR} ${ARGN})
set(${FEATURE_VAR} "TRUE" PARENT_SCOPE)
endfunction(feature_check FEATURE_VAR REQUIRED_VAR)
So, in you case you may use
# In CMakeList.txt, after find_package()
feature_check(USE_DEBUGGER gazebo_FOUND OpenGL_FOUND GLUT_FOUND)
# In debugger/CMakeList.txt
if(NOT USE_DEBUGGER)
return()
endif(NOT USE_DEBUGGER)
...

Answer 2

Turning my comments into an answer
I see three alternatives:
Using FindPkgConfig module to combine the search for packages into a single call/result variable? Something like:
pkg_check_modules(DEBUGGER_DEPS gazebo gl glu)
would - on platforms with pkg-config - also make the find_package() calls obsolete by using the results of the above call:
target_compile_options(debugger PUBLIC ${DEBUGGER_DEPS_CFLAGS})
target_link_libraries(debugger PUBLIC ${DEBUGGER_DEPS_LDFLAGS})
Using CMakeDependentOption module to combine the boolean values into a single option to check with something like
CMAKE_DEPENDENT_OPTION(
BUILD_DEBUGGER "Build debugger" ON
"gazebo_FOUND;OpenGL_FOUND;GLUT_FOUND" OFF
)
Or - if the project is huge - maybe just write my own my_target_link_libraries() function which includes the necessary if (TARGET ...) dependency checks and removes the main target from all/default build if it fails:
function(my_target_link_libraries _target)
foreach(_dep IN ITEMS ${ARGN})
if (TARGET _dep)
target_link_libraries(${_target} ${_dep})
else()
message(STATUS "No target '${_dep}' found. Disabling '${_target}' build")
set_target_properties(${_target} PROPERTIES EXCLUDE_FROM_DEFAULT_BUILD 1)
endif()
endforach()
endfunction()
my_target_link_libraries(debugger GLUT::GLUT ...)