Adds -D define flags to the compilation of source files.

add_definitions(-DFOO -DBAR ...)

Adds definitions to the compiler command line for targets in the current directory and below (whether added before or after this command is invoked). This command can be used to add any flags, but it is intended to add preprocessor definitions (see the add_compile_options() command to add other flags). Flags beginning in -D or /D that look like preprocessor definitions are automatically added to the COMPILE_DEFINITIONS directory property for the current directory. Definitions with non-trivial values may be left in the set of flags instead of being converted for reasons of backwards compatibility. See documentation of the directory, target, source file COMPILE_DEFINITIONS properties for details on adding preprocessor definitions to specific scopes and configurations.

See the cmake-buildsystem(7) manual for more on defining buildsystem properties.