Making BUILD_SHARED_LIBS a CMake GUI option.

Change-Id: I801358cf2e0e8d129c10cf6848ee2f2fa64f75d6

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b144365..b7d2569 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -106,6 +106,12 @@
 OPTION(BUILD_TESTING "Enable tests" ON)
 OPTION(BUILD_DOCUMENTATION "Build User's Guide (html)" OFF)
 OPTION(BUILD_EXAMPLES "Build examples" ON)
+OPTION(BUILD_SHARED_LIBS "Build Ceres as a shared library." OFF)
+IF (BUILD_SHARED_LIBS)
+  MESSAGE("-- Building as a shared library.")
+ELSE (BUILD_SHARED_LIBS)
+  MESSAGE("-- Building as a static library.")
+ENDIF (BUILD_SHARED_LIBS)
 
 # Default locations to search for on various platforms.