Ensure local config.h is used if Ceres is already installed.

- Previously, if Ceres was already installed to the same location as one
  of the depenedencies (e.g. /usr/local) then as the BUILD_DIR/config
  path was necessarily added to the include_directories() list *after*
  the INCLUDE_DIRS for the dependencies, the config.h for the
  *installed* version of Ceres would be used instead of the locally
  configured version, as it would appear first on the path.
- Forcing the location of the locally configured config.h to the front
  of the include_directories() list ensures that it is found first
  (and is thus used) even if an installed version of Ceres also exists
  on the path.

Change-Id: I7b4f97a3ad0120bd11551f9b8eb6bffc66966eb4

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6ae9a5c..4ab2e31 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -775,7 +775,12 @@
 INCLUDE(CreateCeresConfig)
 CREATE_CERES_CONFIG("${CERES_COMPILE_OPTIONS}"
   ${CMAKE_CURRENT_BINARY_DIR}/config/ceres/internal)
-INCLUDE_DIRECTORIES(${CMAKE_CURRENT_BINARY_DIR}/config)
+# Force the location containing the configured config.h to the front of the
+# include_directories list (by default it is appended to the back) to ensure
+# that if the user has an installed version of Ceres in the same location as one
+# of the dependencies (e.g. /usr/local) that we find the config.h we just
+# configured, not the (older) installed config.h.
+INCLUDE_DIRECTORIES(BEFORE ${CMAKE_CURRENT_BINARY_DIR}/config)
 
 ADD_SUBDIRECTORY(internal/ceres)