build: Add missing schnorrsig_halfagg module configuration

configure.ac

@@ -455,6 +455,9 @@ SECP_CFLAGS="$SECP_CFLAGS $WERROR_CFLAGS"
 # Processing must be done in a reverse topological sorting of the dependency graph
 # (dependent module first).
 if test x"$enable_module_schnorrsig_halfagg" = x"yes"; then
+  if test x"$enable_module_schnorrsig" = x"no"; then
+    AC_MSG_ERROR([Module dependency error: You have disabled the schnorrsig module explicitly, but it is required by the schnorrsig_halfagg module.])
+  fi
   SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_SCHNORRSIG_HALFAGG=1"
   enable_module_schnorrsig=yes
 fi