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.

Modules/CheckFunctionExists.cmake

@@ -38,7 +38,7 @@
 
 
 macro(CHECK_FUNCTION_EXISTS FUNCTION VARIABLE)
-  if(NOT DEFINED "${VARIABLE}")
+  if(NOT DEFINED "${VARIABLE}" OR "x${${VARIABLE}}" STREQUAL "x${VARIABLE}")
     set(MACRO_CHECK_FUNCTION_DEFINITIONS
       "-DCHECK_FUNCTION_EXISTS=${FUNCTION} ${CMAKE_REQUIRED_FLAGS}")
     if(NOT CMAKE_REQUIRED_QUIET)

Modules/CheckIncludeFileCXX.cmake

@@ -44,7 +44,7 @@
 #  License text for the above reference.)
 
 macro(CHECK_INCLUDE_FILE_CXX INCLUDE VARIABLE)
-  if(NOT DEFINED "${VARIABLE}")
+  if(NOT DEFINED "${VARIABLE}" OR "x${${VARIABLE}}" STREQUAL "x${VARIABLE}")
     if(CMAKE_REQUIRED_INCLUDES)
       set(CHECK_INCLUDE_FILE_CXX_INCLUDE_DIRS "-DINCLUDE_DIRECTORIES=${CMAKE_REQUIRED_INCLUDES}")
     else()

Modules/CheckSymbolExists.cmake

@@ -50,7 +50,7 @@ macro(CHECK_SYMBOL_EXISTS SYMBOL FILES VARIABLE)
 endmacro()
 
 macro(_CHECK_SYMBOL_EXISTS SOURCEFILE SYMBOL FILES VARIABLE)
-  if(NOT DEFINED "${VARIABLE}")
+  if(NOT DEFINED "${VARIABLE}" OR "x${${VARIABLE}}" STREQUAL "x${VARIABLE}")
     set(CMAKE_CONFIGURABLE_FILE_CONTENT "/* */\n")
     set(MACRO_CHECK_SYMBOL_EXISTS_FLAGS ${CMAKE_REQUIRED_FLAGS})
     if(CMAKE_REQUIRED_LIBRARIES)