Revert "Allow non-absolute values for bootstrap GHC variable"

This broke configure in subtle ways resulting in #25076 where hadrian
didn't end up the boot compiler it was configured to use.

This reverts commit 209d09f5.

configure.ac

@@ -97,11 +97,11 @@ dnl use either is considered a Feature.
 dnl ** What command to use to compile compiler sources ?
 dnl --------------------------------------------------------------
 
-AC_ARG_VAR(GHC,[Use as the bootstrap GHC. [default=autodetect]])
-AC_CHECK_PROG([GHC], [ghc], [ghc])
+AC_ARG_VAR(GHC,[Use as the full path to GHC. [default=autodetect]])
+AC_PATH_PROG([GHC], [ghc])
 AC_ARG_WITH([ghc],
-        AS_HELP_STRING([--with-ghc=PATH], [Use PATH as the bootstrap ghc (obsolete, use GHC=PATH instead) [default=autodetect]]),
-        AC_MSG_ERROR([--with-ghc=$withval is obsolete (use './configure GHC=$withval' instead)]))
+        AS_HELP_STRING([--with-ghc=PATH], [Use PATH as the full path to ghc (obsolete, use GHC=PATH instead) [default=autodetect]]),
+        AC_MSG_ERROR([--with-ghc=$withval is obsolete (use './configure GHC=$withval' or 'GHC=$withval ./configure' instead)]))
 AC_SUBST(WithGhc,$GHC)
 
 AC_ARG_ENABLE(bootstrap-with-devel-snapshot,