Reading the docs it seems clear that this isn't possible.
# Projects may control the use of ARC by defining GS_WITH_ARC=1
# or GS_WITH_ARC=0 in their GNUmakefile, or in the environment,# or as an argument to the 'make' command.
# The default behavior is not to use ARC, unless GNUSTEP_NG_ARC is
# set to 1 (perhaps in the GNUstep config file; GNUstep.conf).
# The value of ARC_OBJCFLAGS is used to specify the flags passed
# to the compiler when building ARC code. If it has not been set,
# it defaults to -fobjc-arc -fobjc-arc-exceptions so that objects
# are not leaked when an exception is raised.
# The value of ARC_CPPFLAGS is used to specify the flags passed
# to the preprocessor when building ARC code. If it has not been set,
# it defaults to -DGS_WITH_ARC=1