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.

1 files changed, 10 insertions, 7 deletions

diff --git a/Help/manual/cmake-language.7.rst b/Help/manual/cmake-language.7.rst
index 71649ba..8740d97 100644
--- a/Help/manual/cmake-language.7.rst
+++ b/Help/manual/cmake-language.7.rst
@@ -206,9 +206,10 @@ enclosed content, such as `Escape Sequences`_ or `Variable References`_,
 is performed.  A bracket argument is always given to the command
 invocation as exactly one argument.
 
-For example:
+.. No code-block syntax highlighting in the following example
+   (long string literal not supported by our cmake.py)
 
-.. code-block:: cmake
+For example::
 
  message([=[
  This is the first line in a bracket argument with bracket length 1.
@@ -253,9 +254,10 @@ closing quotes.  Both `Escape Sequences`_ and `Variable References`_
 are evaluated.  A quoted argument is always given to the command
 invocation as exactly one argument.
 
-For example:
+.. No code-block syntax highlighting in the following example
+   (escape \" not supported by our cmake.py)
 
-::
+For example::
 
  message("This is a quoted argument containing multiple lines.
  This is always one argument even though it contains a ; character.
@@ -264,11 +266,12 @@ For example:
  It does end in an unescaped double quote.
  ")
 
+.. No code-block syntax highlighting in the following example
+   (for conformity with the two above examples)
+
 The final ``\`` on any line ending in an odd number of backslashes
 is treated as a line continuation and ignored along with the
-immediately following newline character.  For example:
-
-.. code-block:: cmake
+immediately following newline character.  For example::
 
  message("\
  This is the first line of a quoted argument. \