cmake: use RbConfig::CONFIG to help identify the compilers

lib/mini_portile2/mini_portile_cmake.rb

@@ -86,11 +86,12 @@ def find_compiler(compilers)
   # configure automatically searches for the right compiler based on the
   # `--host` parameter.  However, CMake doesn't have an equivalent feature.
   # Search for the right compiler for the target architecture using
-  # some basic heruistics.
+  # some basic heuristics.
   def find_c_and_cxx_compilers(host)
-    c_compiler = ENV["CC"]
-    cxx_compiler = ENV["CXX"]
+    c_compiler = ENV["CC"] || RbConfig::CONFIG["CC"]
+    cxx_compiler = ENV["CXX"] || RbConfig::CONFIG["CXX"]
 
+    # TODO: can we remove this if/else block and just rely on RbConfig::CONFIG?
     if MiniPortile.darwin?
       c_compiler ||= 'clang'
       cxx_compiler ||='clang++'