Fix build issues on Darwin.

--- a/devtools/OS/Darwin	2014-03-05 01:59:45.000000000 +0100
+++ b/devtools/OS/Darwin	2020-05-18 14:47:57.000000000 +0200
@@ -8,6 +8,8 @@
 #    We look a lot more like 4.4BSD than NeXTStep or OpenStep.
 #
 define(`confCC', `cc -traditional-cpp -pipe ${Extra_CC_Flags}')
+define(`confCCOPTS_SO', `-fPIC')
+define(`confSOEXT', `dylib')
 define(`confMAPDEF', `-DNEWDB -DNIS -DMAP_REGEX -DNETINFO -DAUTO_NETINFO_ALIASES -DAUTO_NETINFO_HOSTS')
 define(`confENVDEF', `-DDARWIN')
 define(`confLDOPTS', `${Extra_LD_Flags}')
--- a/sendmail/sendmail.h	2020-05-18 14:51:17.000000000 +0200
+++ b/sendmail/sendmail.h	2020-05-18 14:51:00.000000000 +0200
@@ -122,7 +122,11 @@
 # endif
 
 #if NAMED_BIND
-# include <arpa/nameser.h>
+# ifdef __APPLE__
+#  include <arpa/nameser_compat.h>
+# else
+#  include <arpa/nameser.h>
+# endif
 # ifdef NOERROR
 #  undef NOERROR		/* avoid <sys/streams.h> conflict */
 # endif