empty PREFIX leads to wrong path in INTERFACE_INCLUDE_DIRECTORIES on CMake

[dkroenke]

Required information

Operating system:
E.g. Ubuntu 22.04

CMake version:
CMake 3.22.1

Observed result or behaviour:
The INTERFACE_INCLUDE_DIRECTORIES defined in the generated iceoryx_poshTargets.cmake
file contains invalid directory set to ${_IMPORT_PREFIX}/include//iceoryx/v2.90.0.
This affects all iceoryx packages.

When an external application tries to use the INTERFACE_INCLUDE_DIRECTORIES property,
the includes cannot be found.

test code in iceoryx example:

set(HOOFS_LIB)
get_target_property(HOOFS_LIB iceoryx_hoofs::iceoryx_hoofs INTERFACE_INCLUDE_DIRECTORIES)
message("HOOFS_LIB: " ${HOOFS_LIB})

output in out-of-tree build:

HOOFS_LIB: /home/my/folder/iceoryx/build/install/prefix/include//iceoryx/v2.90.0

Expected result or behaviour:

INTERFACE_INCLUDE_DIRECTORIES evaluates always to a valid directory
with set/unset PREFIX variable.

Conditions where it occurred / Performed steps:

The reason lies in this code snippet:

if(PREFIX STREQUAL "")
  set(PREFIX "${_unique_dir}")
else()
  set(PREFIX "${PREFIX}/${_unique_dir}")
endif()

The PREFIX expression evaluates here always to false even when the variable
is not set which is the case in iceoryx.

This leads to an attached / character
in the install include directories.
The generated include directory seems not to be affected by this.

Solution could be to evaluate the actual content of the Variable:

if("${PREFIX}" STREQUAL "")

FYI @sloretz