cmake: change include guard

cmake-format will indent the body of the include guard. Use a
style which does not put the body inside the guard.
https://cmake.org/Wiki/CMake_Performance_Tips#Use_an_include_guard

Rename cbuild/cmake/aom_config.c.cmake to .template to simplify
upcoming formatting.

Change-Id: I70724683d5e581f1b4f69418ed185e0b92df27c3

aom_scale/aom_scale.cmake

diff --git a/aom_scale/aom_scale.cmake b/aom_scale/aom_scale.cmake
index 1c00b59..d9c7a71 100644
--- a/aom_scale/aom_scale.cmake
+++ b/aom_scale/aom_scale.cmake
@@ -8,7 +8,9 @@
 ## Media Patent License 1.0 was not distributed with this source code in the
 ## PATENTS file, you can obtain it at www.aomedia.org/license/patent.
 ##
-if (NOT AOM_AOM_SCALE_AOM_SCALE_CMAKE_)
+if (AOM_AOM_SCALE_AOM_SCALE_CMAKE_)
+  return()
+endif ()  # AOM_AOM_SCALE_AOM_SCALE_CMAKE_
 set(AOM_AOM_SCALE_AOM_SCALE_CMAKE_ 1)
 
 set(AOM_SCALE_SOURCES
@@ -35,5 +37,3 @@
 
   set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_scale PARENT_SCOPE)
 endfunction ()
-
-endif ()  # AOM_AOM_SCALE_AOM_SCALE_CMAKE_