Properly warn when CMake buildfiles are used

CMakeLists.txt

@@ -24,12 +24,17 @@ option(USE_BUNDLED_LIBEVENT "Use the bundled version of spdlog." ${HUNTER_ENABLE
 option(USE_BUNDLED_LIBCURL "Use the bundled version of spdlog." ${HUNTER_ENABLED})
 option(USE_BUNDLED_SPDLOG "Use the bundled version of spdlog." ${HUNTER_ENABLED})
 
-
 project(coeurl
 	VERSION 0.3.0
 	DESCRIPTION "Simple library to do http requests asynchronously via CURL in C++."
 	HOMEPAGE_URL "https://nheko.im/nheko-reborn/cocurl")
 
+if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
+	message(STATUS ${CMAKE_SOURCE_DIR})
+	message(STATUS ${CMAKE_CURRENT_SOURCE_DIR})
+	message(FATAL_ERROR "The CMake build for coeurl is not intended for installation or usage outside of FetchContent.")
+endif()
+
 add_library(coeurl lib/client.cpp lib/request.cpp lib/errors.cpp)
 add_library(coeurl::coeurl ALIAS coeurl)
 target_include_directories(coeurl PUBLIC