cmake: add ENABLE_STATIC_LIB option to build static lib

When using the ENABLE_LIB_ONLY option, only the shared
library was built. This new option allows to build the
static library as well.

lib/CMakeLists.txt

@@ -49,7 +49,7 @@ target_include_directories(nghttp2 INTERFACE
     "${CMAKE_CURRENT_SOURCE_DIR}/includes"
     )
 
-if(HAVE_CUNIT)
+if(HAVE_CUNIT OR ENABLE_STATIC_LIB)
   # Static library (for unittests because of symbol visibility)
   add_library(nghttp2_static STATIC ${NGHTTP2_SOURCES})
   set_target_properties(nghttp2_static PROPERTIES
@@ -58,6 +58,10 @@ if(HAVE_CUNIT)
     ARCHIVE_OUTPUT_NAME nghttp2
   )
   target_compile_definitions(nghttp2_static PUBLIC "-DNGHTTP2_STATICLIB")
+  if(ENABLE_STATIC_LIB)
+    install(TARGETS nghttp2_static
+      DESTINATION "${CMAKE_INSTALL_LIBDIR}")
+  endif()
 endif()
 
 install(TARGETS nghttp2