CMake: Build against "system" libraries in separate prefix

Here again something I noticed when trying to build AppImageKit in a minimal environment using a prefix.
During build time I get this:
```
+ objcopy --add-section .upd_info=1024_blank_bytes --set-section-flags .upd_info=noload,readonly runtime.o runtime2.o
+ objcopy --add-section .sha256_sig=1024_blank_bytes --set-section-flags .sha256_sig=noload,readonly runtime2.o runtime3.o
+ squashfuse_libs='/home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit-build/squashfuse-prefix/src/squashfuse/.libs/libsquashfuse.a;/home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit-build/squashfuse-prefix/src/squashfuse/.libs/libsquashfuse_ll.a;/home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit-build/squashfuse-prefix/src/squashfuse/.libs/libfuseprivate.a'
+ clang -Os -ffunction-sections -fdata-sections -s -Wl,--gc-sections -o runtime /home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit/src/elf.c /home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit/src/notify.c /home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit/src/getsection.c runtime3.o /home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit-build/squashfuse-prefix/src/squashfuse/.libs/libsquashfuse.a /home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit-build/squashfuse-prefix/src/squashfuse/.libs/libsquashfuse_ll.a /home/gitlab-runner/builds/aec59c0c/0/Cura/cura-build-environment/build/AppImageKit-prefix/src/AppImageKit-build/squashfuse-prefix/src/squashfuse/.libs/libfuseprivate.a -L -Wl,-Bdynamic -lpthread -lz -Wl,-Bstatic -llzma -Wl,-Bdynamic -ldl
/usr/bin/ld: cannot find -lz
/usr/bin/ld: cannot find -llzma
clang: error: linker command failed with exit code 1 (use -v to see invocation)
+ cleanup
+ '[' -d /tmp/tmp.b4s3pmgZyS ']'
+ rm -r /tmp/tmp.b4s3pmgZyS
src/CMakeFiles/appimagetool.dir/build.make:65: recipe for target 'src/runtime' failed
make[5]: *** [src/runtime] Error 1
```

I'm wondering how it comes, that both are found during `cmake ..` execution, but during a build, it fails.

The full log is attached.
[raw.txt](https://github.com/AppImage/AppImageKit/files/1758288/raw.txt)