@glaringlee  Thank you for your help! Unfortunately I have the same problem after removing the set(CMAKE_MODULE_PATH… command. This is how my CMakeLists.txt file looks like right now. I have created a simple test program from the cppdocs where I just create a Tensor and print it out with std::cout.
cmake_minimum_required(VERSION 3.0 FATAL_ERROR)
project(libtorch_cross_test)
list(APPEND CMAKE_PREFIX_PATH "/home/marcel/Documents/libs/libtorch1.5linux/libtorch")
find_package(Torch REQUIRED)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${TORCH_CXX_FLAGS}")

add_executable(example-app main.cpp)
target_link_libraries(example-app "${TORCH_LIBRARIES}")
set_property(TARGET example-app PROPERTY CXX_STANDARD 14)

If I compile this without  the cmake toolchain file for the cross compilation everything works just fine.
As soon as I add the cmake flag
-DCMAKE_TOOLCHAIN_FILE=…/cmake/qnx_7.0.0_linux_x86_64.cmake
to use my toolchain I get the same missing: TORCH_LIBRARY message.
cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/qnx_7.0.0_linux_x86_64.cmake ..
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /home/marcel/qnx700/host/linux/x86_64/usr/bin/ntox86_64-gcc
-- Check for working C compiler: /home/marcel/qnx700/host/linux/x86_64/usr/bin/ntox86_64-gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /home/marcel/qnx700/host/linux/x86_64/usr/bin/ntox86_64-c++
-- Check for working CXX compiler: /home/marcel/qnx700/host/linux/x86_64/usr/bin/ntox86_64-c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - found
-- Found Threads: TRUE  
-- Could NOT find torch (missing: TORCH_LIBRARY) 
CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
torch;torch_library;C10_LIBRARY
    linked by target "example-app" in directory /home/marcel/Documents/libtorch_cross_test

-- Configuring incomplete, errors occurred!
See also "/home/marcel/Documents/libtorch_cross_test/build/CMakeFiles/CMakeOutput.log".

The QNX compiler (qcc) seems to be recognized just fine by cmake so I cant explain why this happens.