Check*: Restore support for arbitrary result variable names

In commit db76876db5 (Modules: Use new SOURCES_FROM_* try_compile (1/2),
2022-09-26, v3.25.0-rc1~74^2~1) and commit 41f7b1169a (Modules: Use new
SOURCES_FROM_* try_compile (2/2), 2022-09-26, v3.25.0-rc1~74^2) the
switch to `SOURCE_FROM_*` required a stronger precondition (the second
argument to check_include_files must not have path components) than
before (any variable name could be used).

Fix that by transforming the variable name to a C identifier before
feeding it to try_compile as a filename.  The filename is unspecified by
the documentation, and the file itself is only temporary, so that should
work fine.

I have gone through all the occurrences of `SOURCE_FROM_*`, and
identified these two that require changes.  The rest should work fine as
the filenames do not depend on input variable names.

Fixes: #24204

Modules/CheckIncludeFiles.cmake

@@ -70,10 +70,11 @@ macro(CHECK_INCLUDE_FILES INCLUDE VARIABLE)
       message(FATAL_ERROR "Unknown arguments:\n  ${ARGN}\n")
     endif()
 
+    string(MAKE_C_IDENTIFIER ${VARIABLE} _variable_escaped)
     if(_lang STREQUAL "C")
-      set(src ${VARIABLE}.c)
+      set(src ${_variable_escaped}.c)
     elseif(_lang STREQUAL "CXX")
-      set(src ${VARIABLE}.cpp)
+      set(src ${_variable_escaped}.cpp)
     else()
       message(FATAL_ERROR "Unknown language:\n  ${_lang}\nSupported languages: C, CXX.\n")
     endif()

Modules/CheckTypeSize.cmake

@@ -103,10 +103,11 @@ function(__check_type_size_impl type var map builtin language)
   endif()
 
   # Perform language check
+  string(MAKE_C_IDENTIFIER ${var} _var_escaped)
   if(language STREQUAL "C")
-    set(src ${var}.c)
+    set(src ${_var_escaped}.c)
   elseif(language STREQUAL "CXX")
-    set(src ${var}.cpp)
+    set(src ${_var_escaped}.cpp)
   else()
     message(FATAL_ERROR "Unknown language:\n  ${language}\nSupported languages: C, CXX.\n")
   endif()