This patch changes the default value of 'dict-dir' to correspond
to ~/.guix-profile/lib/aspell rather than $prefix/lib/aspell-X.Y.

This is not strictly necessary for the 'aspell' program itself since
one can simply set "ASPELL_CONF=dict-dir $HOME/.guix-profile/lib/aspell".
However it is necessary for applications that use libaspell since
'ASPELL_CONF' is not honored in this case.  See <https://bugs.gnu.org/25836>.

--- a/common/config.cpp
+++ b/common/config.cpp
@@ -1349,6 +1349,9 @@ namespace acommon {
 #  define REPL     ".aspell.<lang>.prepl"
 #endif
 
+#undef DICT_DIR
+#define DICT_DIR "<$ASPELL_DICT_DIR|home-dir/.guix-profile/lib/aspell>"
+
   static const KeyInfo config_keys[] = {
     // the description should be under 50 chars
     {"actual-dict-dir", KeyInfoString, "<dict-dir^master>", 0}