Linking against an ExternalProject_add dependency in CMAKE

I'm getting this ninja build error below while running Ninja. My CMAKE build command is`cmake -G"Ninja" -DCMAKE_BUILD_TYPE=Release`. ``` ninja: error: 'ext_deps/api/src/ext_api/build/src/lib/libapi.a', needed by 'Project', missing and no known rule to make it ``` Let's say my project consists of an API (downloaded via CMAKE from GitHub) and the implementation (the Project). ``` The layout would look like: Project/ -- build/ -- cmake/modules ----- ExternalDep.cmake ----- FindAPI.cmake -- CMakeLists.txt -- src/ ---- CMakeLists.txt -- include/ ``` Let's say that in the top-level `CMakeLists.txt` I do the usual business of setting build settings, CXX flags, et cetera, and then I call `include(ExternalDep)`, which checks if the "API" library is in the user's system (if not it is downloaded via CMAKE). In `src/CMakeLists.txt` I try to link against the API library using a ``` target_link_libraries(${PROJECT_NAME} PRIVATE ${API_LIBRARY}) ``` The first issue I'm having is that before the "API" library can even be downloaded and built, I get the ninja build error I posted above. I'm positive the `ExternalDep.cmake` is included before I try to add the Project executable and link against the "API" library. Here's a simplified version of `ExternalDep.cmake`: ``` set(EXT_DEPS_PREFIX "ext_deps") ExternalProject_Add(ext_lib GIT_REPOSITORY "https://github.com/fake/api.git" GIT_TAG "master" PREFIX "${CMAKE_BINARY_DIR}/${EXT_DEPS_PREFIX}/api" TMP_DIR "${CMAKE_BINARY_DIR}/${EXT_DEPS_PREFIX}/api-tmp" STAMP_DIR "${CMAKE_BINARY_DIR}/${EXT_DEPS_PREFIX}/api-stamp" CMAKE_ARGS -DCMAKE_BUILD_TYPE=Release SOURCE_DIR "${CMAKE_BINARY_DIR}/${EXT_DEPS_PREFIX}/api/ext_api" BINARY_DIR "${CMAKE_BINARY_DIR}/${EXT_DEPS_PREFIX}/api/ext_api-build" BUILD_ALWAYS true TEST_COMMAND "") add_dependencies(ext_projects ext_api) set(API_LIBRARY "${CMAKE_BINARY_DIR}/${EXT_DEPS_PREFIX}/api/ext_api-build/src/lib/libapi.a") ```