There was already a perfectly good README-BCMATH, I just found the different naming confusing.

README.bcmath

@@ -1,6 +0,0 @@
-NOTE: If you are going to build with --enable-bcmath, you must download
-http://www.php.net/version4/do_download.php?download_file=number.tar.gz
-and unpack it in the top php4 directory.
-
-PHP no longer ships the bcmath files with the distribution because of
-licensing restrictions.

ext/bcmath/config.m4

@@ -10,6 +10,13 @@ AC_ARG_ENABLE(bcmath,
     AC_DEFINE(WITH_BCMATH, 1, [Whether you have bcmath])
     AC_MSG_RESULT(yes)
     PHP_EXTENSION(bcmath)
+	if test -f $srcdir/ext/bcmath/number.c; then
+		if grep "Dummy File" $srcdir/ext/bcmath/number.c >/dev/null; then
+		  AC_MSG_ERROR(You do not have the bcmath package. Please read the README-BCMATH.)
+		fi
+	else
+		AC_MSG_ERROR(You do not have the bcmath package. Please read the README-BCMATH.)
+	fi
   else
     AC_DEFINE(WITH_BCMATH, 0,[ ])
     AC_MSG_RESULT(no)
@@ -18,3 +25,7 @@ AC_ARG_ENABLE(bcmath,
   AC_DEFINE(WITH_BCMATH, 0,[ ])
   AC_MSG_RESULT(no)
 ]) 
+
+dnl ## Local Variables:
+dnl ## tab-width: 4
+dnl ## End: