cmake: add PARALLEL_COMPILE_JOBS option to project CMakeLists.txt

this allows to limit the number of compile jobs to e.g. one or two
which is needed when using parallel build systems like Ninja on
resource (RAM) contrained systems, like the RPi4

CMakeLists.txt

@@ -95,6 +95,13 @@ else(${CMAKE_SYSTEM_PROCESSOR} MATCHES "aarch64")
   set(GCC_ARCH native CACHE STRING "GCC compile for specific architecture.")
 endif(${CMAKE_SYSTEM_PROCESSOR} MATCHES "aarch64")
 
+# On RAM constrained (embedded) systems it may be useful to limit parallel compilation with, e.g. -DPARALLEL_COMPILE_JOBS=1
+if (PARALLEL_COMPILE_JOBS)
+  set(CMAKE_JOB_POOL_COMPILE compile_job_pool${CMAKE_CURRENT_SOURCE_DIR})
+  string (REGEX REPLACE "[^a-zA-Z0-9]+" "_" CMAKE_JOB_POOL_COMPILE ${CMAKE_JOB_POOL_COMPILE})
+  set_property(GLOBAL APPEND PROPERTY JOB_POOLS ${CMAKE_JOB_POOL_COMPILE}=${PARALLEL_COMPILE_JOBS})
+  message(STATUS "${CMAKE_CURRENT_SOURCE_DIR}: Limiting compiler jobs to ${PARALLEL_COMPILE_JOBS}")
+endif ()
 
 if (ENABLE_SRSLOG_TRACING)
   add_definitions(-DENABLE_SRSLOG_EVENT_TRACE)