build: disable optional use of gmp in internal secp256k1 build

Until secp256k1 is used for verification there is no reason for Bitcoin
Core's secp256k1 to link against gmp, even if available. Pass a flag to
configure to override the bignum implementation.

This fixes a crash at runtime on ppc64 reported by @gmaxwell.

configure.ac

@@ -885,7 +885,7 @@ PKGCONFIG_LIBDIR_TEMP="$PKG_CONFIG_LIBDIR"
 unset PKG_CONFIG_LIBDIR
 PKG_CONFIG_LIBDIR="$PKGCONFIG_LIBDIR_TEMP"
 
-ac_configure_args="${ac_configure_args} --disable-shared --with-pic"
+ac_configure_args="${ac_configure_args} --disable-shared --with-pic --with-bignum=no"
 AC_CONFIG_SUBDIRS([src/secp256k1])
 
 AC_OUTPUT