Consider the following: I have a project which depends on libwebsockets which is itself dependends on mbedtls. I pull everything in from github using FetchContent, which establishes the dependencies in the build folder (_deps).
As I want to build everything statically, I thought I might do libwebsockets a favor and setup CMAKE_PREFIX_PATH so that its find_path() calls will find mbedtls, so libwebsockets knows the path to mbedtls and can see its include files. At the same time I have set cache variables that libwebsockets uses in its find_library() calls variables to the targets in mbedtls, so they are always found even at configure time. More specifically, this is what libwebsocket does:
find_path(LWS_MBEDTLS_INCLUDE_DIRS mbedtls/ssl.h)
find_library(MBEDTLS_LIBRARY mbedtls)
find_library(MBEDX509_LIBRARY mbedx509)
find_library(MBEDCRYPTO_LIBRARY mbedcrypto)
So libwebsockets finds mbedtls in the build directory, everything good so far, but then I get this nasty error still at configure time:
CMake Error in build/_deps/libwebsockets-src/lib/CMakeLists.txt:
Target "websockets" INTERFACE_INCLUDE_DIRECTORIES property contains path:
"C:/dev/realtime-cpp/build/_deps/mbedtls-build/include"
which is prefixed in the build directory.
There are similar errors for the other targets. I understand that the include files are in the build directory. However I don't understand why this is a problem and what I should do instead?