build: Clean up handling of module dependencies

This also makes the order in which module options are processed
consistent between CMake and autotools (the reverse order of the listing
printed to stdout).

configure.ac

@@ -387,27 +387,27 @@ SECP_CFLAGS="$SECP_CFLAGS $WERROR_CFLAGS"
 ### Handle module options
 ###
 
-if test x"$enable_module_ecdh" = x"yes"; then
-  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_ECDH=1"
+# Processing must be done in a reverse topological sorting of the dependency graph
+# (dependent module first).
+if test x"$enable_module_ellswift" = x"yes"; then
+  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_ELLSWIFT=1"
+fi
+
+if test x"$enable_module_schnorrsig" = x"yes"; then
+  enable_module_extrakeys=yes
+  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_SCHNORRSIG=1"
+fi
+
+if test x"$enable_module_extrakeys" = x"yes"; then
+  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_EXTRAKEYS=1"
 fi
 
 if test x"$enable_module_recovery" = x"yes"; then
   SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_RECOVERY=1"
 fi
 
-if test x"$enable_module_schnorrsig" = x"yes"; then
-  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_SCHNORRSIG=1"
-  enable_module_extrakeys=yes
-fi
-
-if test x"$enable_module_ellswift" = x"yes"; then
-  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_ELLSWIFT=1"
-fi
-
-# Test if extrakeys is set after the schnorrsig module to allow the schnorrsig
-# module to set enable_module_extrakeys=yes
-if test x"$enable_module_extrakeys" = x"yes"; then
-  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_EXTRAKEYS=1"
+if test x"$enable_module_ecdh" = x"yes"; then
+  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_ECDH=1"
 fi
 
 if test x"$enable_external_default_callbacks" = x"yes"; then