Stops the use of QT5 excecutable compile flags when building shared librari...

My earlier pull request fixed the compile time error caused by line 44. On Linux it adds the -fPIE flag to CMAKE_CXX_FLAGS. When building shared libraries it will eventually cause the linker to fail with:

```
Linking CXX shared library ../lib/libpcl_common.so
/usr/bin/ld: CMakeFiles/pcl_common.dir/src/io.cpp.o: relocation R_X86_64_PC32 against symbol `_Z9fieldCompPKN3pcl13PCLPointFieldES2_' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status
make[2]: *** [lib/libpcl_common.so.1.7.0] Error 1
make[1]: *** [common/CMakeFiles/pcl_common.dir/all] Error 2
```

Without line 44, everything links fine. The flag doesn't really make sense unless you are building executables.