[cmake] Fix CMake build on newer CMake versions

Unfortunately, newer CMake versions die during regex variable extraction, causing the build to fail.

This is caused by the lack of escaping used around variables in the extract_make_variable function, causing these variables to be automatically unwrapped into empty strings.

CMakeLists.txt

@@ -142,8 +142,8 @@ endif ()
 
 ## Extract variables from Makefile files
 function (extract_make_variable variable makefile_source)
-  string(REGEX MATCH "${variable} = ([^$]+)\\$" temp ${makefile_source})
-  string(REGEX MATCHALL "[^ \n\t\\]+" listVar ${CMAKE_MATCH_1})
+  string(REGEX MATCH "${variable} = ([^$]+)\\$" temp "${makefile_source}")
+  string(REGEX MATCHALL "[^ \n\t\\]+" listVar "${CMAKE_MATCH_1}")
   set (${variable} ${listVar} PARENT_SCOPE)
 endfunction ()