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.

(cherry picked from commit b7bcf729)

configure.ac

@@ -726,9 +726,6 @@ 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
-