autoconf: remove unused context diff check

This patch removes redundant autoconf check for the context diff
program given it isn't actually been used anywhere, especially since
make removal.

configure.ac

@@ -709,9 +709,6 @@ AC_SUBST(TargetHasIdentDirective)
 GHC_GNU_NONEXEC_STACK
 AC_SUBST(TargetHasGnuNonexecStack)
 
-dnl ** figure out how to do context diffs
-FP_PROG_CONTEXT_DIFF
-
 dnl Let's make sure install-sh is executable here.  If we got it from
 dnl a darcs repo, it might not be (see bug #978).
 chmod +x install-sh

m4/fp_prog_context_diff.m4

-# FP_PROG_CONTEXT_DIFF
-# --------------------
-# Figure out how to do context diffs. Sets the output variable ContextDiffCmd.
-#
-# Note: NeXTStep thinks diff'ing a file against itself is "trouble".
-AC_DEFUN([FP_PROG_CONTEXT_DIFF],
-[AC_CACHE_CHECK([for a working context diff], [fp_cv_context_diff],
-[echo foo > conftest1
-echo foo > conftest2
-fp_cv_context_diff=no
-for fp_var in '-U 1' '-u1' '-C 1' '-c1'
-do
-  if diff $fp_var conftest1 conftest2 > /dev/null 2>&1; then
-    fp_cv_context_diff="diff $fp_var"
-    break
-  fi
-done])
-if test x"$fp_cv_context_diff" = xno; then
-   AC_MSG_ERROR([cannot figure out how to do context diffs])
-fi
-AC_SUBST(ContextDiffCmd, [$fp_cv_context_diff])
-])# FP_PROG_CONTEXT_DIFF
-