swift_build_support/cmake.py: parallelize default CMake build (#66212)

CMake's bootstrapping script can't infer a number of parallel build jobs from the number of CPU cores on its own, which means that it's not parallelized by default. Let's get the value of CPU cores when `build_jobs` argument is not explicitly set.

`make -j` only has an effect on CMake's post-bootstrap build jobs, while a substantial amount of time was spent building code sequentially in CMake's bootstrap script without that `--parallelize` option, which is called on a line preceding the `make -j` invocation.

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/cmake.py

@@ -15,6 +15,7 @@
 # ----------------------------------------------------------------------------
 
 
+import multiprocessing
 import os
 import platform
 import re
@@ -273,9 +274,11 @@ def build_cmake(self, source_root, build_root):
 
         cwd = os.getcwd()
         os.chdir(cmake_build_dir)
-        shell.call_without_sleeping([cmake_bootstrap, '--no-qt-gui', '--',
-                                    '-DCMAKE_USE_OPENSSL=OFF'], echo=True)
-        shell.call_without_sleeping(['make', '-j%s' % self.args.build_jobs],
+        build_jobs = self.args.build_jobs or multiprocessing.cpu_count()
+        shell.call_without_sleeping([cmake_bootstrap, '--no-qt-gui',
+                                     '--parallel=%s' % build_jobs, '--',
+                                     '-DCMAKE_USE_OPENSSL=OFF'], echo=True)
+        shell.call_without_sleeping(['make', '-j%s' % build_jobs],
                                     echo=True)
         os.chdir(cwd)
         return os.path.join(cmake_build_dir, 'bin', 'cmake')