[CMake] linking: absolute path vs -l<libname>

[Nico Schlömer]

Hi everyone,

I'm curious about when CMake decides to link a library by its absolute path
and when it links using the `-l*` syntax. I came across this for the very
simple test problem
```
cmake_minimum_required(VERSION 3.0)

project(mytest)

find_package(ZLIB REQUIRED)
find_package(netCDF REQUIRED)

add_executable(mytest main.cpp)
target_link_libraries(
  mytest
  ${ZLIB_LIBRARIES}
  ${netCDF_LIBRARIES}
  )
```
The resulting link line is
```
/usr/bin/c++       CMakeFiles/mytest.dir/main.cpp.o  -o mytest -rdynamic
-lz /usr/lib/x86_64-linux-gnu/libnetcdf.so.7.3.0
```
so ZLIB is linked with `-lz`, netCDF with the full absolute path.

Seems inconsistent? (ZLIB by the way is found through CMake's
FindZLIB.cmake, netCDF by their own export files.)

So far, I had been under the impression that `-l*` defeats the purpose of
CMake a little bit as it asks the linker to search in the system's paths
for a libz.so, something CMake has already done.

A little clarification here would be great.

Cheers,
Nico
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