Adding librt to link libraries for SuiteSparse_config on Linux.

- SuiteSparse_config (SuiteSparse >= 4.0) requires librt if compiled
  with timing support, which is the default on Linux & Unix (except
  OSX).
- This was previously raised as issue 124:
  https://code.google.com/p/ceres-solver/issues/detail?id=124.

Change-Id: I08ada41cd28d2e63a4bf342e143e23fe8213e9ee

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b7d2569..e54c40c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -298,8 +298,23 @@
   MESSAGE("-- Did not find SuiteSparse_config header")
 ENDIF (EXISTS ${SUITESPARSE_CONFIG_INCLUDE})
 
-IF (NOT EXISTS ${SUITESPARSE_CONFIG_LIB} OR
-    NOT EXISTS ${SUITESPARSE_CONFIG_INCLUDE})
+IF (EXISTS ${SUITESPARSE_CONFIG_LIB} AND
+    EXISTS ${SUITESPARSE_CONFIG_INCLUDE})
+  # SuiteSparse_config (SuiteSparse version >= 4) requires librt library for
+  # timing by default when compiled on Linux or Unix, but not on OSX (which
+  # does not have librt).
+  IF (CMAKE_SYSTEM_NAME MATCHES "Linux" OR UNIX AND NOT APPLE)
+    FIND_LIBRARY(LIBRT_LIB NAMES rt)
+    IF (LIBRT_LIB)
+      MESSAGE("-- Adding librt: ${LIBRT_LIB} to SuiteSparse_config libraries.")
+    ELSE (LIBRT_LIB)
+      MESSAGE("-- Could not find librt, but found SuiteSparse_config, "
+        "assuming that SuiteSparse was compiled without timing.")
+    ENDIF (LIBRT_LIB)
+    LIST(APPEND SUITESPARSE_CONFIG_LIB ${LIBRT_LIB})
+  ENDIF (CMAKE_SYSTEM_NAME MATCHES "Linux" OR UNIX AND NOT APPLE)
+ELSE (EXISTS ${SUITESPARSE_CONFIG_LIB} AND
+      EXISTS ${SUITESPARSE_CONFIG_INCLUDE})
   SET(SUITESPARSE_CONFIG_FOUND FALSE)
   FIND_PATH(UFCONFIG_INCLUDE NAMES UFconfig.h)
   IF (EXISTS ${UFCONFIG_INCLUDE})
@@ -308,8 +323,8 @@
     MESSAGE("-- Did not find UFconfig header")
     SET(UFCONFIG_FOUND FALSE)
   ENDIF (EXISTS ${UFCONFIG_INCLUDE})
-ENDIF (NOT EXISTS ${SUITESPARSE_CONFIG_LIB} OR
-       NOT EXISTS ${SUITESPARSE_CONFIG_INCLUDE})
+ENDIF (EXISTS ${SUITESPARSE_CONFIG_LIB} AND
+       EXISTS ${SUITESPARSE_CONFIG_INCLUDE})
 
 FIND_LIBRARY(METIS_LIB NAMES metis)
 IF (EXISTS ${METIS_LIB})

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index 7c23a68..477cf98 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -154,9 +154,9 @@
   LIST(APPEND CERES_LIBRARY_DEPENDENCIES ${CAMD_LIB})
   LIST(APPEND CERES_LIBRARY_DEPENDENCIES ${COLAMD_LIB})
   LIST(APPEND CERES_LIBRARY_DEPENDENCIES ${AMD_LIB})
-  IF (EXISTS ${SUITESPARSE_CONFIG_LIB})
+  IF (SUITESPARSE_CONFIG_FOUND)
     LIST(APPEND CERES_LIBRARY_DEPENDENCIES ${SUITESPARSE_CONFIG_LIB})
-  ENDIF (EXISTS ${SUITESPARSE_CONFIG_LIB})
+  ENDIF (SUITESPARSE_CONFIG_FOUND)
 
   IF (EXISTS ${METIS_LIB})
     LIST(APPEND CERES_LIBRARY_DEPENDENCIES ${METIS_LIB})