[CMake] an akward preprocessor

[luxInteg]

Greetings
I am learning cmake

I have a small project     to be installed in some directory $INSTALLED 
=/whatever/installed/directory/is


I have some files say  fila1.c ..File2.c  to compile   with a preprocessor 
that includes a reference to  $INSTALL
how do I set  the compile properties of files with this?

do I do :- 
-DWITH_INSTALL_DIR or 
-Dwhatever_installed_directory_is (i.e. for example -D/usr/local)  
or ?????


advice would be appreciated

sincerely
luxInteg