Compiler and linker arguments set for a specific subproject, for example -Dsub:c_args=-DFOO on the command line or c_args in the default_options of a subproject() call, are now added to the compile and link commands of that subproject's targets. Previously such values were accepted and stored but silently ignored.
Similarly, <lang>_args and <lang>_link_args entries in a target's override_options now take effect.
In all cases the per-subproject or per-target value replaces the global value (including flags coming from environment variables such as CFLAGS), it is not appended to it. This matches what get_option('c_args') already returned inside a subproject.