Based on the FreeBSD patch that brings the generic C99 interface to all
architectures, this patch makes sure that "special" arch code isn't used.

--- gsl-2.4/ieee-utils/fp.c	2015-11-03 10:21:55.000000000 -0600
+++ gsl-2.4/ieee-utils/fp.c	2018-04-24 22:45:00.934921840 -0500
@@ -1,6 +1,8 @@
 #include <config.h>
 
-#if HAVE_GNUSPARC_IEEE_INTERFACE
+#if HAVE_DECL_FEENABLEEXCEPT || HAVE_DECL_FESETTRAPENABLE
+#include "fp-gnuc99.c"
+#elif HAVE_GNUSPARC_IEEE_INTERFACE
 #include "fp-gnusparc.c"
 #elif HAVE_GNUM68K_IEEE_INTERFACE
 #include "fp-gnum68k.c"
@@ -43,8 +45,6 @@
 # else
 #  include "fp-darwin86.c"
 #endif
-#elif HAVE_DECL_FEENABLEEXCEPT || HAVE_DECL_FESETTRAPENABLE
-#include "fp-gnuc99.c"
 #else
 #include "fp-unknown.c" 
 #endif