report error when --enable-extflonums fails

The `--enable-extflonums` option doesn't really do anything, since
extflonum support is enabled automtatically when the compiler's
configuration allows it. To make this slightly less confusing, report
an error when extflonums cannot be supported, despite
`--enable-extflonums`. The error is reported via compiling, instead of
via `configure`, but hopefully that's enough to be helpful.

racket/src/configure

@@ -6583,6 +6583,11 @@ $as_echo "#define MZ_NO_EXTFLONUMS 1" >>confdefs.h
   fi
 fi
 
+if test "${enable_extflonum}" = "yes" ; then
+  $as_echo "#define MZ_INSIST_EXTFLONUMS 1" >>confdefs.h
+
+fi
+
 ############## libffi ################
 
 # Depends on pthread on some platforms

racket/src/racket/configure.ac

@@ -1475,6 +1475,10 @@ if test "${enable_extflonum}" = "default" ; then
   fi
 fi
 
+if test "${enable_extflonum}" = "yes" ; then
+  AC_DEFINE(MZ_INSIST_EXTFLONUMS)
+fi
+
 ############## libffi ################
 
 # Depends on pthread on some platforms

racket/src/racket/include/scheme.h

@@ -93,6 +93,9 @@ typedef struct {
 typedef long double mz_long_double;
 # endif
 #else
+# ifdef MZ_INSIST_EXTFLONUMS
+#  error "cannot support extflonums; you may need to adjust compiler options"
+# endif
 typedef double mz_long_double;
 #endif
 

racket/src/racket/mzconfig.h.in

@@ -95,6 +95,9 @@ typedef unsigned long uintptr_t;
 /* To disable extflonums when they would otherwise work: */
 #undef MZ_NO_EXTFLONUMS
 
+/* Extflonums are specifically requested (so complain if not supported): */
+#undef MZ_INSIST_EXTFLONUMS
+
 /* Library subpath */
 #undef SPLS_SUFFIX