CMake building targets conditionally based on library existence

I have a large cross-platform project which needs to build in various places; in some places, different UI toolkits, sound APIs, etc. may be available, and I am trying to figure out the best way to automatically configure which targets get configured based on which libraries are present. The code I am trying for that is, for example: ``` find_library(PC_EGL EGL) find_library(PC_GLESv2 GLESv2) find_library(PC_Xxf86vm Xxf86vm) if (DEFINED PC_EGL AND DEFINED PC_GLESv2 AND DEFINED PC_Xxf86vm) add_executable(foo foo.cpp) target_link_libraries(foo ${PC_EGL} ${PC_GLESv2} ${PC_Xxf86vm}) endif() ``` However, in the case that I build this on a system which doesn't have libGLESv2 available, I get the error: ``` CMake Error: The following variables are used in this project, but they are set to NOTFOUND. Please set them or make sure they are set and tested correctly in the CMake files: PC_GLESv2 linked by target "foo" in directory /path/to/platform ``` The find_library documentation implies that the variable PC_EGL_NOTFOUND should be getting set, but it isn't (CMake 2.8.5). So, what is the appropriate way to use find_library to determine whether a target should be made to exist at all? It seems like using ``` if (NOT PC_EGL MATCH "-NOTFOUND") ``` is a bit fragile and fiddly, so is there a better mechanism for determining a CMake command path based on wheter a library was found at all?