Added additional test for gcvt as uClibc appears to have this and not
ecvt.

diff --git a/configure.ac b/configure.ac
index b3cfe9b..ad433ea 100644
--- a/configure.ac
+++ b/configure.ac
@@ -916,10 +916,12 @@
 	chsize \
 	clone \
 	ecvt \
+	fcvt \
 	finite \
 	fpclass \
 	ftruncate \
 	ftruncate64 \
+	gcvt \
 	getnetbyaddr \
 	getnetbyname \
 	getopt_long \