Q: Cmake GNUInstallDirs variables empty When trying to compile my project using cmake ., variable CMAKE_INSTALL_LIBDIR is empty and I have no idea why.
I'm trying to issue the following (on line 40):
install(TARGETS fpthread
    EXPORT fpthread_config
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})

I get the following error message:
CMake Error at CMakeLists.txt:40 (install):
  install TARGETS given no LIBRARY DESTINATION for shared library target
  "fpthread".

Issuing the following results in an empty string:
MESSAGE(STATUS ${CMAKE_INSTALL_LIBDIR})

I've tried both Cmake 3.5 (installed through package manager) and Cmake 3.11 (latest release, compiled from source).

A: Reformulating my previous comment as answer:
To access variables from the GNUInstallDirs module you need to add
include(GNUInstallDirs)

to your CMakeLists.txt file. The module is not included by default.
