Help: Factor out COMPILE_DEFINITIONS disclaimer duplication

The COMPILE_DEFINITIONS escaping disclaimer was represented in builtin
documentation using a preprocessor macro.  Factor the duplicate content
out into a separate .txt file and include it in each document with the
reStructuredText include directive.

Help/include/COMPILE_DEFINITIONS_DISCLAIMER.txt

@@ -0,0 +1,18 @@
+Disclaimer: Most native build tools have poor support for escaping
+certain values.  CMake has work-arounds for many cases but some values
+may just not be possible to pass correctly.  If a value does not seem
+to be escaped correctly, do not attempt to work-around the problem by
+adding escape sequences to the value.  Your work-around may break in a
+future version of CMake that has improved escape support.  Instead
+consider defining the macro in a (configured) header file.  Then
+report the limitation.  Known limitations include::
+
+  #          - broken almost everywhere
+  ;          - broken in VS IDE 7.0 and Borland Makefiles
+  ,          - broken in VS IDE
+  %          - broken in some cases in NMake
+  & |        - broken in some cases on MinGW
+  ^ < > \"   - broken in most Make tools on Windows
+
+CMake does not reject these values outright because they do work in
+some cases.  Use with caution.