Explicitly set --exists as the default option

This happened basically by accident before when "pkg-config foo" was run
because the code wouldn't find any options set and just fall through to
the end after processing the package arguments. However, it would act
differently in that Requires.private was only enabled with an explicit
--exists.

check/check-missing

@@ -51,10 +51,10 @@ EXPECT_RETURN=0
 RESULT="/usr/include/somedir"
 run_test --variable includedir missing-requires-private
 
-# tests below are on an existing package, but with missing Requires; when
-# pkg-config outputs error, the actual error text isn't checked
+# tests below are on an existing package, but with missing Requires;
+# when pkg-config outputs error, the actual error text isn't checked
 # package exists
-EXPECT_RETURN=0
+EXPECT_RETURN=1
 RESULT=""
 run_test missing-requires
 

main.c

@@ -529,6 +529,13 @@ main (int argc, char **argv)
       return 1;
     }
 
+  /* If no output option was set, then --exists is the default. */
+  if (!output_opt_set)
+    {
+      debug_spew ("no output option set, defaulting to --exists\n");
+      want_exists = TRUE;
+    }
+
   /* Error printing is determined as follows:
    *     - for --cflags, --libs, etc. it's on by default
    *       and --silence-errors can turn it off

pkg-config.1

@@ -193,7 +193,8 @@ constraint after each package name, for example:
 .nf
   $ pkg-config --exists 'glib-2.0 >= 1.3.4 libxml = 1.8.3'
 .fi
-Remember to use \-\-print-errors if you want error messages.
+Remember to use \-\-print-errors if you want error messages. When no
+output options are supplied to \fIpkg-config\fP, \-\-exists is implied.
 .TP
 .I "--msvc-syntax"
 This option is available only on Windows. It causes \fIpkg-config\fP