CMakeDetermineCCompiler: Fix typo "_CXX_" -> "_C_" (#13330)

Initialize CMAKE_C_COMPILER_INIT before using it, rather than
CMAKE_CXX_COMPILER_INIT.

Modules/CMakeDetermineCCompiler.cmake

@@ -32,7 +32,7 @@
 #   _CMAKE_TOOLCHAIN_PREFIX
 
 IF(NOT CMAKE_C_COMPILER)
-  SET(CMAKE_CXX_COMPILER_INIT NOTFOUND)
+  SET(CMAKE_C_COMPILER_INIT NOTFOUND)
 
   # prefer the environment variable CC
   IF($ENV{CC} MATCHES ".+")