Link BLAS explicitly on non-Mac platforms

Fixes issue #3.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a8a5c63..70336ae 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -145,6 +145,18 @@
   ENDIF (NOT EXISTS ${LAPACK_LIB})
   MESSAGE ("-- Found LAPACK library: ${LAPACK_LIB}")
 
+  # On Apple BLAS is linked with vecLib, so don't add it separately.
+  IF (NOT APPLE)
+    MESSAGE("-- Check for BLAS")
+    FIND_LIBRARY(BLAS_LIB NAMES blas)
+    IF (NOT EXISTS ${BLAS_LIB})
+      MESSAGE(FATAL_ERROR
+              "Can't find BLAS (needed for LAPACK and SuiteSparse.)"
+              "Please specify -DBLAS_LIB=...")
+    ENDIF (NOT EXISTS ${BLAS_LIB})
+    MESSAGE ("-- Found BLAS library: ${BLAS_LIB}")
+  ENDIF (NOT APPLE)
+
 ELSE (SUITESPARSE)
   ADD_DEFINITIONS(-DCERES_NO_SUITESPARSE)
 ENDIF (SUITESPARSE)

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index 1601666..f02db23 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -119,6 +119,12 @@
       ${METIS_LIB})
 ENDIF (EXISTS ${METIS_LIB})
 
+IF (EXISTS ${BLAS_LIB})
+  SET(CERES_LIBRARY_DEPENDENCIES
+      ${CERES_LIBRARY_DEPENDENCIES}
+      ${BLAS_LIB})
+ENDIF (EXISTS ${BLAS_LIB})
+
 IF (OPENMP_FOUND)
   SET(CERES_LIBRARY_DEPENDENCIES
     ${CERES_LIBRARY_DEPENDENCIES}