Help: Override pygments CMakeLexer to support <..> and [..]

* The code snippets in the docs consist of CMake code mixed
  with syntax definition punctuation like < > [ ] ... Therefore
  a pure CMake lexer is inadequate. Here it is replaced by a
  CMake syntax definition parser.
* Fixed syntax definition snippets in FindPkgConfig.cmake to
  make best use of syntax highlighting. This source file is the
  hardest to support because it contains comparison operators
  <= = >=, which need special attention to avoid confusion
  with the placeholder indicators <...>.
* Fixed syntax in execute_process.rst (there were unbalanced
  brackets).
* Disabled syntax highlighting for long string examples in
  cmake-language.7.rst.
* No highlighting of removed syntax in CMP0049
* To inspect the outcome of this patch, see e.g. the pages
  * manual/cmake-buildsystem.7.html
  * module/ExternalProject.html
  * module/FindPkgConfig.html
  which are particularly rich in complex code snippets.

Help/command/execute_process.rst

@@ -5,8 +5,8 @@ Execute one or more child processes.
 
 .. code-block:: cmake
 
-  execute_process(COMMAND <cmd1> [args1...]]
-                  [COMMAND <cmd2> [args2...] [...]]
+  execute_process(COMMAND <cmd1> [<arguments>]
+                  [COMMAND <cmd2> [<arguments>]]...
                   [WORKING_DIRECTORY <directory>]
                   [TIMEOUT <seconds>]
                   [RESULT_VARIABLE <variable>]