setup: do not require __MRC__ defined for Mac OS 9 builds

Partially reverts "somewhat protect Mac OS X users from using Mac OS 9
config file", commit 62519bfe05.

Do things that are specific to classic Mac OS (i.e. include config-mac.h
in curl_setup.h and rename "main" to "curl_main" in tool_setup.h) when
only "macintosh" is defined. Remove the additional condition that
"__MRC__" should be defined since that would only be true with the MPW
MrC compiler which prevents the use of other reasonable compilers like
the MPW SC compiler and especially the Metrowerks CodeWarrior compilers.
"macintosh" is only defined by classic Mac OS compilers so this change
should not affect users of Mac OS X / OS X / macOS / any other OS.

Closes #10037

lib/curl_setup.h

@@ -92,7 +92,7 @@
 #  endif
 #endif
 
-#if defined(macintosh) && defined(__MRC__)
+#ifdef macintosh
 #  include "config-mac.h"
 #endif
 

src/tool_setup.h

@@ -47,7 +47,7 @@
  * Platform specific stuff.
  */
 
-#if defined(macintosh) && defined(__MRC__)
+#ifdef macintosh
 #  define main(x,y) curl_main(x,y)
 #endif