are all options in CMakelists.txt necessary?

i edited file CMakeLists.txt given at  http://pointclouds.org/documentation/tutorials/using_pcl_pcl_config.php#using-pcl-pcl-config

i removed the line

link_directories(${PCL_LIBRARY_DIRS})

the code still compiled well in console & eclipse.It ran fine in both.how is this possible?

it could be possible that cmake looks for link directories in some default places & if libraries are found in these default locations then, the code is compiled & run even if the link directories are missing.


when i ran the cmake xaple form cmake tutorial at: http://web.cs.swarthmore.edu/~adanner/tips/cmake.php


i removed the link_libraries option & i got the following error

[ 60%] Building CXX object CMakeFiles/oglfirst.dir/oglfirst.o
Linking CXX executable oglfirst
CMakeFiles/oglfirst.dir/oglfirst.o:oglfirst.cpp:function display(): error: undefined reference to 'glClear'

....
collect2: ld returned 1 exit status
make[2]: *** [oglfirst] Error 1
make[1]: *** [CMakeFiles/oglfirst.dir/all] Error 2