build-llvm.py: Add defines options

Give users the flexibility to define the values that they want so that
it is not necessary to continue to pollute the script with various
flags.

For example, if a user wants to limit the number of compile and link
jobs that ninja can do at one time:

$ ./build-llvm.py -D LLVM_PARALLEL_COMPILE_JOBS=2 LLVM_PARALLEL_LINK_JOBS=2

Closes: #132
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

build-llvm.py

@@ -143,6 +143,18 @@ def parse_parameters(root_folder):
                         """),
                         type=str,
                         default="ClangBuiltLinux")
+    parser.add_argument("-D",
+                        "--defines",
+                        help=textwrap.dedent("""\
+                        Specify additional cmake values. These will be applied to all cmake invocations.
+
+                        Example: -D LLVM_PARALLEL_COMPILE_JOBS=2 LLVM_PARALLEL_LINK_JOBS=2
+
+                        See https://llvm.org/docs/CMake.html for various cmake values. Note that some of
+                        the options to this script correspond to cmake values.
+
+                        """),
+                        nargs="+")
     parser.add_argument("-i",
                         "--incremental",
                         help=textwrap.dedent("""\
@@ -895,6 +907,9 @@ def invoke_cmake(args, dirs, env_vars, stage):
     for key in defines:
         newdef = '-D' + key + '=' + defines[key]
         cmake += [newdef]
+    if args.defines:
+        for d in args.defines:
+            cmake += ['-D' + d]
     cmake += [dirs.root_folder.joinpath("llvm-project", "llvm").as_posix()]
 
     cwd = dirs.build_folder.joinpath("stage%d" % stage).as_posix()