Fix two build errors.

1. Not set -march=native -mtune=native for GCC versions less than 4.2
2. Explicit cast in lapack.cc

Thanks to Dustin Lang for reporting this.

Change-Id: I71fbefa5f7db0188516f9f1dc044d7d5f374e2cd

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8cb4a7c..7bb4a39 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -600,7 +600,9 @@
   IF (CMAKE_COMPILER_IS_GNUCXX)
     # Linux
     IF (CMAKE_SYSTEM_NAME MATCHES "Linux")
-      SET (CERES_CXX_FLAGS "${CERES_CXX_FLAGS} -march=native -mtune=native")
+      IF (NOT GCC_VERSION VERSION_LESS 4.2)
+        SET (CERES_CXX_FLAGS "${CERES_CXX_FLAGS} -march=native -mtune=native")
+      ENDIF (NOT GCC_VERSION VERSION_LESS 4.2)
     ENDIF (CMAKE_SYSTEM_NAME MATCHES "Linux")
     # Mac OS X
     IF (CMAKE_SYSTEM_NAME MATCHES "Darwin")

internal/ceres/lapack.cc

diff --git a/internal/ceres/lapack.cc b/internal/ceres/lapack.cc
index 73bfa69..e93e05f 100644
--- a/internal/ceres/lapack.cc
+++ b/internal/ceres/lapack.cc
@@ -114,7 +114,7 @@
          &info);
 
   CHECK_EQ(info, 0);
-  return work;
+  return static_cast<int>(work);
 #endif
 }