Add new feature: != shell assignment for portability with BSD make.

Feature submitted by David Wheeler.

variable.c

@@ -1111,6 +1111,29 @@ set_special_var (struct variable *var)
   return var;
 }
 
+/* Given a string, shell-execute it and return a malloc'ed string of the
+ * result. This removes only ONE newline (if any) at the end, for maximum
+ * compatibility with the *BSD makes.  If it fails, returns NULL. */
+
+char *
+shell_result (const char *p)
+{
+  char *buf;
+  unsigned int len;
+  char *args[2];
+  char *result;
+
+  install_variable_buffer (&buf, &len);
+
+  args[0] = (char *) p;
+  args[1] = NULL;
+  variable_buffer_output (func_shell_base (variable_buffer, args, 0), "\0", 1);
+  result = strdup (variable_buffer);
+
+  restore_variable_buffer (buf, len);
+  return result;
+}
+
 /* Given a variable, a value, and a flavor, define the variable.
    See the try_variable_definition() function for details on the parameters. */
 
@@ -1140,6 +1163,16 @@ do_variable_definition (const struct floc *flocp, const char *varname,
          target-specific variable.  */
       p = alloc_value = allocated_variable_expand (value);
       break;
+    case f_shell:
+      {
+        /* A shell definition "var != value".  Expand value, pass it to
+           the shell, and store the result in recursively-expanded var. */
+        char *q = allocated_variable_expand (value);
+        p = alloc_value = shell_result (q);
+        free (q);
+        flavor = f_recursive;
+        break;
+      }
     case f_conditional:
       /* A conditional variable definition "var ?= value".
          The value is set IFF the variable is not defined yet. */
@@ -1432,7 +1465,7 @@ parse_variable_definition (const char *p, enum variable_flavor *flavor)
 	  return (char *)p;
 	}
 
-      /* Match assignment variants (:=, +=, ?=)  */
+      /* Match assignment variants (:=, +=, ?=, !=)  */
       if (*p == '=')
         {
           switch (c)
@@ -1446,6 +1479,9 @@ parse_variable_definition (const char *p, enum variable_flavor *flavor)
               case '?':
                 *flavor = f_conditional;
                 break;
+              case '!':
+                *flavor = f_shell;
+                break;
               default:
                 /* If we skipped whitespace, non-assignments means no var.  */
                 if (wspace)