You have one extra level of nesting.
CMAKE will search under $CMAKE_PREFIX_PATH/include
for headers and $CMAKE_PREFIX_PATH/libs
for libraries.
From CMAKE documentation:
For each path in the CMAKE_PREFIX_PATH list, CMake will check "PATH/include" and "PATH" when FIND_PATH() is called, "PATH/bin" and "PATH" when FIND_PROGRAM() is called, and "PATH/lib and "PATH" when FIND_LIBRARY() is called.