docs: Replace references to configure.in with configure.ac

docs/README-macosx.md

@@ -100,7 +100,7 @@ You should replace EXE_NAME with the name of the executable. APP_NAME is what
 will be visible to the user in the Finder. Usually it will be the same
 as EXE_NAME but capitalized. E.g. if EXE_NAME is "testgame" then APP_NAME 
 usually is "TestGame". You might also want to use `@PACKAGE@` to use the package
-name as specified in your configure.in file.
+name as specified in your configure.ac file.
 
 If your project builds more than one application, you will have to do a bit
 more. For each of your target applications, you need a separate rule.

docs/README-porting.md

@@ -13,7 +13,7 @@ There are two basic ways of building SDL at the moment:
 
 1. The "UNIX" way:  ./configure; make; make install
 
-   If you have a GNUish system, then you might try this.  Edit configure.in,
+   If you have a GNUish system, then you might try this.  Edit configure.ac,
    take a look at the large section labelled:
 
 	"Set up the configuration based on the host platform!"