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

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 62519bfe059251af2914199f284c736553ff0489.

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