Install static library.

Closes jgm/CommonMark#381.
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index f619453..f3e9877 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -117,9 +117,10 @@
 
 set(CMAKE_INSTALL_SYSTEM_RUNTIME_LIBS_NO_WARNINGS ON)
 include (InstallRequiredSystemLibraries)
-install(TARGETS ${PROGRAM} ${LIBRARY}
+install(TARGETS ${PROGRAM} ${LIBRARY} ${STATICLIBRARY}
   RUNTIME DESTINATION bin
   LIBRARY DESTINATION lib
+  ARCHIVE DESTINATION lib
   )
 
 install(FILES