ci: limit builds through the machine load

This allows other jobs to cooperatively schedule with a build.

.gitlab/ci/ctest_build.cmake

@@ -6,6 +6,20 @@ ctest_read_custom_files("${CTEST_BINARY_DIRECTORY}")
 # Pick up from where the configure left off.
 ctest_start(APPEND)
 
+include(ProcessorCount)
+ProcessorCount(nproc)
+if (NOT "$ENV{CTEST_MAX_PARALLELISM}" STREQUAL "")
+  if (nproc GREATER "$ENV{CTEST_MAX_PARALLELISM}")
+    set(nproc "$ENV{CTEST_MAX_PARALLELISM}")
+  endif ()
+endif ()
+
+if (CTEST_CMAKE_GENERATOR STREQUAL "Unix Makefiles")
+  set(CTEST_BUILD_FLAGS "-j${nproc} -l${nproc}")
+elseif (CTEST_CMAKE_GENERATOR MATCHES "Ninja")
+  set(CTEST_BUILD_FLAGS "-l${nproc}")
+endif ()
+
 set(targets_to_build "all")
 
 foreach (target IN LISTS targets_to_build)
@@ -16,7 +30,7 @@ foreach (target IN LISTS targets_to_build)
 
   if (CTEST_CMAKE_GENERATOR MATCHES "Make")
     # Drop the `-i` flag without remorse.
-    set(CTEST_BUILD_COMMAND "make -j 4 ${CTEST_BUILD_FLAGS}")
+    set(CTEST_BUILD_COMMAND "make ${CTEST_BUILD_FLAGS}")
 
     if (NOT target STREQUAL "all")
       set(CTEST_BUILD_COMMAND "${CTEST_BUILD_COMMAND} ${target}")