math/nauty: fix build on powerpc

Works around the clang crash, which seems to be fixed in the new snapshots:
PLEASE submit a bug report to https://bugs.freebsd.org/submit/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: clang -cc1 -triple powerpc-unknown-freebsd14.2 -emit-obj -O2 -vectorize-loops gutil2-502595.c
1.      <eof> parser at end of file
2.      Optimizer
Segmentation fault (core dumped)

math/nauty/Makefile

@@ -31,6 +31,8 @@ TEST_TARGET=	check
 
 OPTIONS_DEFINE=	DOCS
 
+CFLAGS_powerpc=	-O1
+
 post-build: # build shared libraries: based on how static libraries are created
 	# rebuild .o files with -fPIC
 	@cd ${WRKSRC} ; \