Check*: Allow result variables to contain regex special characters (#14923)

Prior to the existence of the if(DEFINED) condition, many of our Check
modules implemented the condition with a hack that takes advantage of
the auto-dereference behavior of the if() command to detect if a
variable is defined.  The hack has the form:

 if("${VAR} MATCHES "^${VAR}$")

where "${VAR}" is a macro argument reference.  However, this does not
work when the variable named in the macro argument contains characters
that have special meaning in regular expressions, such as '+'.  Run the
command

 git grep -E 'if\("\$\{.*\}" MATCHES "\^\$\{.*\}\$"\)' -- Modules/Check*

to identify lines with this problem.  Use if(NOT DEFINED) instead.

Modules/CheckIncludeFiles.cmake

@@ -39,7 +39,7 @@
 #  License text for the above reference.)
 
 macro(CHECK_INCLUDE_FILES INCLUDE VARIABLE)
-  if("${VARIABLE}" MATCHES "^${VARIABLE}$")
+  if(NOT DEFINED "${VARIABLE}")
     set(CMAKE_CONFIGURABLE_FILE_CONTENT "/* */\n")
     if(CMAKE_REQUIRED_INCLUDES)
       set(CHECK_INCLUDE_FILES_INCLUDE_DIRS "-DINCLUDE_DIRECTORIES=${CMAKE_REQUIRED_INCLUDES}")