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/CheckCSourceCompiles.cmake

@@ -39,7 +39,7 @@
 
 
 macro(CHECK_C_SOURCE_COMPILES SOURCE VAR)
-  if("${VAR}" MATCHES "^${VAR}$")
+  if(NOT DEFINED "${VAR}")
     set(_FAIL_REGEX)
     set(_key)
     foreach(arg ${ARGN})

Modules/CheckCSourceRuns.cmake

@@ -39,7 +39,7 @@
 
 
 macro(CHECK_C_SOURCE_RUNS SOURCE VAR)
-  if("${VAR}" MATCHES "^${VAR}$")
+  if(NOT DEFINED "${VAR}")
     set(MACRO_CHECK_FUNCTION_DEFINITIONS
       "-D${VAR} ${CMAKE_REQUIRED_FLAGS}")
     if(CMAKE_REQUIRED_LIBRARIES)

Modules/CheckCXXSourceCompiles.cmake

@@ -39,7 +39,7 @@
 
 
 macro(CHECK_CXX_SOURCE_COMPILES SOURCE VAR)
-  if("${VAR}" MATCHES "^${VAR}$")
+  if(NOT DEFINED "${VAR}")
     set(_FAIL_REGEX)
     set(_key)
     foreach(arg ${ARGN})

Modules/CheckCXXSourceRuns.cmake

@@ -39,7 +39,7 @@
 
 
 macro(CHECK_CXX_SOURCE_RUNS SOURCE VAR)
-  if("${VAR}" MATCHES "^${VAR}$")
+  if(NOT DEFINED "${VAR}")
     set(MACRO_CHECK_FUNCTION_DEFINITIONS
       "-D${VAR} ${CMAKE_REQUIRED_FLAGS}")
     if(CMAKE_REQUIRED_LIBRARIES)

Modules/CheckFunctionExists.cmake

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

Modules/CheckIncludeFile.cmake

@@ -40,7 +40,7 @@
 #  License text for the above reference.)
 
 macro(CHECK_INCLUDE_FILE INCLUDE VARIABLE)
-  if("${VARIABLE}" MATCHES "^${VARIABLE}$")
+  if(NOT DEFINED "${VARIABLE}")
     if(CMAKE_REQUIRED_INCLUDES)
       set(CHECK_INCLUDE_FILE_C_INCLUDE_DIRS "-DINCLUDE_DIRECTORIES=${CMAKE_REQUIRED_INCLUDES}")
     else()

Modules/CheckIncludeFileCXX.cmake

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

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}")

Modules/CheckLibraryExists.cmake

@@ -41,7 +41,7 @@
 
 
 macro(CHECK_LIBRARY_EXISTS LIBRARY FUNCTION LOCATION VARIABLE)
-  if("${VARIABLE}" MATCHES "^${VARIABLE}$")
+  if(NOT DEFINED "${VARIABLE}")
     set(MACRO_CHECK_LIBRARY_EXISTS_DEFINITION
       "-DCHECK_FUNCTION_EXISTS=${FUNCTION} ${CMAKE_REQUIRED_FLAGS}")
     if(NOT CMAKE_REQUIRED_QUIET)

Modules/CheckSymbolExists.cmake

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

Modules/CheckVariableExists.cmake

@@ -45,7 +45,7 @@
 
 
 macro(CHECK_VARIABLE_EXISTS VAR VARIABLE)
-  if("${VARIABLE}" MATCHES "^${VARIABLE}$")
+  if(NOT DEFINED "${VARIABLE}")
     set(MACRO_CHECK_VARIABLE_DEFINITIONS
       "-DCHECK_VARIABLE_EXISTS=${VAR} ${CMAKE_REQUIRED_FLAGS}")
     if(NOT CMAKE_REQUIRED_QUIET)