CMake can not determine linker language for target: fileloader

I know there are already a few threads on this topic, however, after reading through many of them I have been unable to find a solution to my problem. I am working on a file loader/parser and am using CMake for the first time. My CMakeList.txt file is being used to import an XML parser (xerces) and currently looks like:

cmake_minimum_required(VERSION 2.8)
project(fileloader)

set(SRCS
        Source.cpp
)

include_directories(./
        ${SPF_ROOT}/xerces/win64/include/xercesc/dom
)

add_executables(${PROJECT_NAME} ${SRCS})

add_library(HEADER_ONLY_TARGET STATIC XMLString.hpp XercesDOMParser.hpp DOM.hpp HandlerBase.hpp PlatformUtils.hpp)
set_target_properties(HEADER_ONLY_TARGET PROPERTIES LINKER_LANGUAGE CXX)

I am relatively new to c++ and completely new to CMake so hopefully I am missing something simple, but any and all help is greatly appreciated!