GNUInstallDirs should be included before usage of CMAKE_INSTALL_INCLUDEDIR

[ruslo]

If it's included too late, the interface directory will not be populated.

Currently:

$ cmake -B _builds -D CMAKE_INSTALL_PREFIX=${HOME}/opt/sioclient
$ cmake --build _builds --target install

$ grep INTERFACE_INCLUDE_DIRECTORIES ${HOME}/opt/sioclient/lib/cmake/sioclient/sioclientTargets.cmake || echo FAIL
FAIL

As a workaround, CMAKE_INSTALL_INCLUDEDIR can be added explicitly:

$ cmake -B _builds -D CMAKE_INSTALL_PREFIX=${HOME}/opt/sioclient -D CMAKE_INSTALL_INCLUDEDIR=include
$ cmake --build _builds --target install

$ grep INTERFACE_INCLUDE_DIRECTORIES ${HOME}/opt/sioclient/lib/cmake/sioclient/sioclientTargets.cmake || echo FAIL
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"

With this patch:

$ cmake -B _builds -D CMAKE_INSTALL_PREFIX=${HOME}/opt/sioclient
$ cmake --build _builds --target install

$ grep INTERFACE_INCLUDE_DIRECTORIES ${HOME}/opt/sioclient/lib/cmake/sioclient/sioclientTargets.cmake || echo FAIL
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"

[jmigual]

Wow so, weird. Thank you for fixing this!

[jmigual]

As soon as the build succeeds I'll merge.