Check*: Add compatibility hack for old VXL code

The cleanup in commit 4f2fcce4 (Check*: Allow result variables to
contain regex special characters, 2014-07-31) broke old VXL code that
abuses the old "if(MATCHES)" implementation by using

  SET( ${VARIABLE} ${VARIABLE} )

to reset a check result variable.  Add a compatibility hack to the
CheckFunctionExists, CheckIncludeFileCXX, and CheckSymbolExists modules
to re-run their checks when the result variable is set to its own name.
Use STREQUAL instead of MATCHES so that special characters still work.
Merge topic 'fix-check-module-DEFINED-tests' into next

4ffec8f6 Check*: Add compatibility hack for old VXL code