Apparently didn't commit the change to the configure script after
updating configure.ac in the commit two days ago.

scripts/configure.ac

diff --git a/scripts/configure.ac b/scripts/configure.ac
index 30c6caa..f5d43e8 100755
--- a/scripts/configure.ac
+++ b/scripts/configure.ac
@@ -6,6 +6,14 @@
 AM_PATH_PYTHON([3.4])
 AX_PYTHON_MODULE([distutils],[])
 
+# Check for Mac OS X gsed vs. sed
+m4_ifndef([AC_PROG_SED],
+    [AC_DEFUN([AC_PROG_SED],[AC_ARG_VAR([SED])
+	AC_CHECK_PROGS([SED],[gsed sed])])])
+
+AC_PROG_SED
+AC_SUBST(SED)
+
 realpath() {
     # Cross-platform, handles ~
     python3 -c 'import os,sys;print(os.path.realpath(os.path.expanduser(sys.argv@<:@1@:>@)))' $1