Removes duplicated include_directories() entry

${CMAKE_CURRENT_SOURCE_DIR} and ${CMAKE_SOURCE_DIR}/libfm-qt have the same
content.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Splits the checks for needed libraries

libfm and libmenu-cache are developed as being part of LXQt. When we want
to use the bundled versions we have to trick pkg_check_modules(). In this
scenario is usefull to have this split.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>