You can override the system defaults by set()ting different values in three places, listed here in increasing priority (i.e., later settings overwrite earlier ones):

The current package's CMakeLists.txt (i.e., the content *preceding* the rosbuild_init() invocation)
If present, $(ROS_ROOT)/rosconfig.cmake
If present, rosconfig.cmake in the current package's top-level directory

But ROS_ROOT/core/rosbuild/rosconfig.cmake says:

3 # To change your build configuration, create a file called

4 # 'rosconfig.cmake' and put it where it will be loaded from here.

5 # The order of processing is the following, with later steps overriding

6 # earlier ones:

7 # - read this file

8 # - if present, read $(ROS_ROOT)/rosconfig.cmake

9 # - if present, read rosconfig.cmake from the current packages's top-level

10 # directory.

11 # - read the rest of the package's CMakeLists.txt (i.e., the content after

12 # the invocation of rospack(), which is how we get here