fix regression in precomputing CMAKE_SIZEOF_VOID_P

In commit 808d5934dd, compiler.sizeof was
refactored to introduce caching, but cmake subprojects did not adapt to
that API change and ended up embedding the python repr of a tuple as a
cmake variable.

mesonbuild/modules/cmake.py

@@ -267,7 +267,7 @@ class CmakeModule(ExtensionModule):
         if not compiler:
             raise mesonlib.MesonException('Requires a C or C++ compiler to compute sizeof(void *).')
 
-        return compiler.sizeof('void *', '', env)
+        return compiler.sizeof('void *', '', env)[0]
 
     def detect_cmake(self, state):
         if self.cmake_detected: