buildsys: make it possible to specify aspell prefix (for macOS)

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>

doc/INSTALL

@@ -89,9 +89,10 @@ incomplete, use `configure --help' to get the full list):
      built-in file editor.  The built-in editor is compiled in by
      default.
 
-`--enable-aspell'
-     This option adds spell check support in the internal editor using
-     libaspell. Disabled by default.
+`--enable-aspell[=prefix]'
+     This option adds spell check support to the internal editor using
+     libaspell and optionally sets path to libaspell installation prefix
+     [default=/usr]. Disabled by default.
 
 `--without-gpm-mouse'
      Use this flag to disable gpm mouse support (e.g. if you want to

m4.include/mc-with-internal-edit.m4

@@ -18,11 +18,14 @@ AC_DEFUN([mc_WITH_INTERNAL_EDIT], [
 
     dnl ASpell support.
     AC_ARG_ENABLE([aspell],
-        AS_HELP_STRING([--enable-aspell], [Enable aspell support for internal editor @<:@no@:>@]),
+        AS_HELP_STRING(
+        [--enable-aspell@<:@=prefix@:>@],
+        [Enable aspell support for internal editor @<:@no@:>@] and optionally set path to aspell installation prefix @<:@default=/usr@:>@),
         [
             if test "x$enableval" = xno; then
                 enable_aspell=no
             else
+                test -d "$enable_aspell/include" && CPPFLAGS="$CPPFLAGS -I$enable_aspell/include"
                 enable_aspell=yes
             fi
         ],
@@ -40,7 +43,7 @@ AC_DEFUN([mc_WITH_INTERNAL_EDIT], [
                 AC_MSG_NOTICE([using aspell for internal editor])
             else
                 enable_aspell=no
-                AC_MSG_NOTICE([aspell support is disabled because gmodule support is not available])
+                AC_MSG_ERROR([aspell support is disabled because gmodule support is not available])
             fi
     fi
 ])