Autodetect gflags namespace.

- At version 2.1, gflags changed from using the google namespace, to
  using gflags by default.  However, it can be configured at build time
  to be something else (which would be google for legacy compatibility
  unless you were evil).
- Ceres previously assumed that gflags was in the google namespace.
- Now, FindGFlags.cmake extracts the namespace when gflags.h is found
  and saves it in GFLAGS_NAMESPACE.
- When building the tests and examples that require gflags,
  CERES_GFLAGS_NAMESPACE is defined to be the detected namespace, and
  all tests/examples now use CERES_GFLAGS_NAMESPACE:: instead of
  google:: when calling gflags functions.

Change-Id: Ia333df7a7e2f08ba9f26bbd339c3a785b88f04c4

cmake/FindGflags.cmake

@@ -36,6 +36,11 @@
 # GFLAGS_FOUND: TRUE iff gflags is found.
 # GFLAGS_INCLUDE_DIRS: Include directories for gflags.
 # GFLAGS_LIBRARIES: Libraries required to link gflags.
+# GFLAGS_NAMESPACE: The namespace in which gflags is defined.  In versions of
+#                   gflags < 2.1, this was google, for versions >= 2.1 it is
+#                   by default gflags, although can be configured when building
+#                   gflags to be something else (i.e. google for legacy
+#                   compatibility).
 #
 # The following variables control the behaviour of this module:
 #
@@ -67,6 +72,7 @@ MACRO(GFLAGS_REPORT_NOT_FOUND REASON_MSG)
   UNSET(GFLAGS_FOUND)
   UNSET(GFLAGS_INCLUDE_DIRS)
   UNSET(GFLAGS_LIBRARIES)
+  UNSET(GFLAGS_NAMESPACE)
   # Make results of search visible in the CMake GUI if gflags has not
   # been found so that user does not have to toggle to advanced view.
   MARK_AS_ADVANCED(CLEAR GFLAGS_INCLUDE_DIR
@@ -128,6 +134,46 @@ ENDIF (NOT GFLAGS_LIBRARY OR
 # if called.
 SET(GFLAGS_FOUND TRUE)
 
+# Identify what namespace gflags was built with.
+IF (GFLAGS_INCLUDE_DIR)
+  # First try the (older) google namespace.
+  INCLUDE(CheckCXXSourceCompiles)
+  # Setup include path & link library for gflags for CHECK_CXX_SOURCE_COMPILES
+  SET(CMAKE_REQUIRED_INCLUDES ${GFLAGS_INCLUDE_DIR})
+  SET(CMAKE_REQUIRED_LIBRARIES ${GFLAGS_LIBRARY})
+  CHECK_CXX_SOURCE_COMPILES(
+    "#include <gflags/gflags.h>
+     int main(int argc, char * argv[]) {
+       google::ParseCommandLineFlags(&argc, &argv, true);
+       return 0;
+     }"
+     GFLAGS_IN_GOOGLE_NAMESPACE)
+  IF (GFLAGS_IN_GOOGLE_NAMESPACE)
+    SET(GFLAGS_NAMESPACE google)
+  ELSE (GFLAGS_IN_GOOGLE_NAMESPACE)
+    # Try (newer) gflags namespace instead.
+    #
+    # Setup include path & link library for gflags for CHECK_CXX_SOURCE_COMPILES
+    SET(CMAKE_REQUIRED_INCLUDES ${GFLAGS_INCLUDE_DIR})
+    SET(CMAKE_REQUIRED_LIBRARIES ${GFLAGS_LIBRARY})
+    CHECK_CXX_SOURCE_COMPILES(
+      "#include <gflags/gflags.h>
+       int main(int argc, char * argv[]) {
+         gflags::ParseCommandLineFlags(&argc, &argv, true);
+         return 0;
+       }"
+       GFLAGS_IN_GFLAGS_NAMESPACE)
+    IF (GFLAGS_IN_GFLAGS_NAMESPACE)
+      SET(GFLAGS_NAMESPACE gflags)
+    ENDIF (GFLAGS_IN_GFLAGS_NAMESPACE)
+  ENDIF (GFLAGS_IN_GOOGLE_NAMESPACE)
+
+  IF (NOT GFLAGS_NAMESPACE)
+    GFLAGS_REPORT_NOT_FOUND(
+      "Failed to determine gflags namespace, it is not google or gflags.")
+  ENDIF (NOT GFLAGS_NAMESPACE)
+ENDIF (GFLAGS_INCLUDE_DIR)
+
 # gflags does not seem to provide any record of the version in its
 # source tree, thus cannot extract version.
 
@@ -162,11 +208,12 @@ ENDIF (GFLAGS_FOUND)
 # Handle REQUIRED / QUIET optional arguments.
 INCLUDE(FindPackageHandleStandardArgs)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(Gflags DEFAULT_MSG
-  GFLAGS_INCLUDE_DIRS GFLAGS_LIBRARIES)
+  GFLAGS_INCLUDE_DIRS GFLAGS_LIBRARIES GFLAGS_NAMESPACE)
 
 # Only mark internal variables as advanced if we found gflags, otherwise
 # leave them visible in the standard GUI for the user to set manually.
 IF (GFLAGS_FOUND)
   MARK_AS_ADVANCED(FORCE GFLAGS_INCLUDE_DIR
-                         GFLAGS_LIBRARY)
+                         GFLAGS_LIBRARY
+                         GFLAGS_NAMESPACE)
 ENDIF (GFLAGS_FOUND)