diff --git a/configure.in b/configure.in
index b4ce50196b49b54b86be8ea3d49e9ee06e4a0564..77c04ac9cbf6fb14f438ce4a86b1e369921a3337 100644
--- a/configure.in
+++ b/configure.in
@@ -9,7 +9,7 @@ AM_INIT_AUTOMAKE(nettle, 1.0)
 
 AM_CONFIG_HEADER(config.h)
 
-dnl Checks for programs.
+# Checks for programs.
 AC_PROG_CC
 AC_PROG_MAKE_SET
 AC_PROG_RANLIB
@@ -21,22 +21,25 @@ fi
 
 AC_PATH_PROG(M4, m4, m4)
 
-dnl Checks for typedefs, structures, and compiler characteristics.
+# Checks for typedefs, structures, and compiler characteristics.
 AC_C_CONST
 AC_C_INLINE
 AC_TYPE_UID_T
 AC_TYPE_SIZE_T
 AC_HEADER_TIME
 
-dnl Needed by the supplied memcmp.c
+# Needed by the supplied memcmp.c
 AC_C_BIGENDIAN
 
-AC_CHECK_SIZEOF(short, 2)
-AC_CHECK_SIZEOF(int, 4)
-AC_CHECK_SIZEOF(long, 4)
-
 AC_REPLACE_FUNCS(memxor)
 
+# Checks for libraries
+AC_CHECK_LIB(gmp, __gmpz_getlimbn,,
+    [AC_MSG_WARN(
+[GNU MP not found, or not 3.1 or up, see http://www.swox.com/gmp.
+Support for public key algorithms will be unavailable.])])
+
+
 # Set these flags *last*, or else the test programs won't compile
 if test x$GCC = xyes ; then
     CFLAGS="$CFLAGS -ggdb3 -Wall -W \