cmake: Add an option to control what files needs to be traced

Even in relatively small projects using `--trace` (and `--trace-expand`)
may produce a lot of output.  When developing a custom module usually
one is interested in output of only a few particular modules.

Add a `--trace-source=<file>` option to enable tracing only a subset of
source files.  The final output would be only from requested modules,
ignoring anything else not matched to given filename(s).

Help/manual/cmake.1.rst

@@ -108,6 +108,11 @@ Options
 
  Like ``--trace``, but with variables expanded.
 
+``--trace-source=<file>``
+ Put cmake in trace mode, but output only lines of a specified file.
+
+ Multiple options are allowed.
+
 ``--warn-uninitialized``
  Warn about uninitialized values.
 

Help/release/dev/cmake-trace-source.rst

@@ -0,0 +1,4 @@
+cmake-trace-source
+------------------
+
+* The :manual:`cmake(1)` command learned a ``--trace-source=<file>`` option.