diff3: use bsd diff3 by default

share/mk/src.opts.mk

@@ -102,7 +102,6 @@ __DEFAULT_YES_OPTIONS = \
     FREEBSD_UPDATE \
     FTP \
     GAMES \
-    GNU_DIFF \
     GOOGLETEST \
     GPIO \
     HAST \
@@ -209,6 +208,7 @@ __DEFAULT_NO_OPTIONS = \
     DTRACE_ASAN \
     DTRACE_TESTS \
     EXPERIMENTAL \
+    GNU_DIFF \
     HESIOD \
     IPFILTER_IPFS \
     LOADER_VERBOSE \