CMake: Further increase RAM threshold for regular compile jobs.

Not great, but the alternative solution (putting several 'main'
targets, like `bf_blenkernel`, into the heavy compile pool, would be
even worse).

CMakeLists.txt

@@ -1827,16 +1827,23 @@ Define the maximum number of concurrent heavy compilation jobs, for ninja build
     set(_compile_heavy_jobs)
     set(_compile_heavy_jobs_max)
 
-    # Heuristics: Assume 3Gb of RAM is needed per regular compile job.
+    # Heuristics: Assume 4Gb of RAM is needed per regular compile job.
     # Typical RAM peak usage of these is actually way less than 1GB usually,
     # but this also accounts for the part of the physical RAM being used by other unrelated
     # processes on the system, and the part being used by the 'heavy' compile and linking jobs.
     #
+    # FIXME:
+    # There are a few files in 'normal' compile job pool now that require a significant amount of RAM
+    # (e.g. `blenkernel/intern/volume.cc` can require almost 5GB of RAM in debug + ASAN builds). Until
+    # we can add individual files to the heavy compile pool job (not possible currently with CMake),
+    # this is the best that can be done. Alternative solution would be to put several whole targets
+    # (like `bf_blenkernel`) into the heavy pool, but that is likely even worse of a workaround.
+    #
     # If there are 'enough' cores available, cap the maximum number of regular jobs to
     # `number of cores - 1`, otherwise allow using all cores if there is enough RAM available.
     # This allows to ensure that the heavy jobs won't get starved by too many normal jobs,
     # since the former usually take a long time to process.
-    math(EXPR _compile_jobs "${_TOT_MEM} / 3000")
+    math(EXPR _compile_jobs "${_TOT_MEM} / 4000")
     if(${_NUM_CORES} GREATER 3)
       math(EXPR _compile_jobs_max "${_NUM_CORES} - 1")
     else()