CMake: Add missing BUILD_SHARED_LIBS option

CMakeLists.txt

@@ -422,9 +422,11 @@ endif()
 option(STATIC "Link libraries statically" ${DEFAULT_STATIC})
 
 # This is a CMake built-in switch that concerns internal libraries
-if (NOT DEFINED BUILD_SHARED_LIBS AND NOT STATIC AND CMAKE_BUILD_TYPE_LOWER STREQUAL "debug")
-    set(BUILD_SHARED_LIBS ON)
+set(BUILD_SHARED_LIBS_DEFAULT OFF)
+if (NOT STATIC AND CMAKE_BUILD_TYPE_LOWER STREQUAL "debug")
+	set(BUILD_SHARED_LIBS_DEFAULT ON)
 endif()
+option(BUILD_SHARED_LIBS "Build internal libraries as shared" ${BUILD_SHARED_LIBS_DEFAULT})
 
 if (BUILD_SHARED_LIBS)
   message(STATUS "Building internal libraries with position independent code")