Do not use "diff" found on PATH while building and installing

Some of the flags used with the first diff found in PATH cause the
vendor diff to choke.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/install-webdoc.sh

@@ -12,7 +12,7 @@ do
 	then
 		: did not match
 	elif test -f "$T/$h" &&
-	   diff -u -I'Last updated [0-9][0-9]-[A-Z][a-z][a-z]-' "$T/$h" "$h"
+	   $DIFF -u -I'Last updated [0-9][0-9]-[A-Z][a-z][a-z]-' "$T/$h" "$h"
 	then
 		:; # up to date
 	else

Makefile

@@ -287,6 +287,7 @@ export prefix bindir sharedir sysconfdir
 CC = gcc
 AR = ar
 RM = rm -f
+DIFF = diff
 TAR = tar
 FIND = find
 INSTALL = install
@@ -1460,7 +1461,7 @@ endif
 ALL_CFLAGS += $(BASIC_CFLAGS)
 ALL_LDFLAGS += $(BASIC_LDFLAGS)
 
-export TAR INSTALL DESTDIR SHELL_PATH
+export DIFF TAR INSTALL DESTDIR SHELL_PATH
 
 
 ### Build rules
@@ -1877,6 +1878,7 @@ GIT-CFLAGS: FORCE
 GIT-BUILD-OPTIONS: FORCE
 	@echo SHELL_PATH=\''$(subst ','\'',$(SHELL_PATH_SQ))'\' >$@
 	@echo PERL_PATH=\''$(subst ','\'',$(PERL_PATH_SQ))'\' >>$@
+	@echo DIFF=\''$(subst ','\'',$(subst ','\'',$(DIFF)))'\' >>$@
 	@echo TAR=\''$(subst ','\'',$(subst ','\'',$(TAR)))'\' >>$@
 	@echo NO_CURL=\''$(subst ','\'',$(subst ','\'',$(NO_CURL)))'\' >>$@
 	@echo NO_PERL=\''$(subst ','\'',$(subst ','\'',$(NO_PERL)))'\' >>$@

config.mak.in

@@ -8,6 +8,7 @@ LDFLAGS = @LDFLAGS@
 CC_LD_DYNPATH = @CC_LD_DYNPATH@
 AR = @AR@
 TAR = @TAR@
+DIFF = @DIFF@
 #INSTALL = @INSTALL@		# needs install-sh or install.sh in sources
 TCLTK_PATH = @TCLTK_PATH@
 

configure.ac

@@ -362,6 +362,7 @@ fi
 #AC_PROG_INSTALL		# needs install-sh or install.sh in sources
 AC_CHECK_TOOLS(AR, [gar ar], :)
 AC_CHECK_PROGS(TAR, [gtar tar])
+AC_CHECK_PROGS(DIFF, [gnudiff gdiff diff])
 # TCLTK_PATH will be set to some value if we want Tcl/Tk
 # or will be empty otherwise.
 if test -z "$NO_TCLTK"; then

git-merge-one-file.sh

@@ -107,7 +107,7 @@ case "${1:-.}${2:-.}${3:-.}" in
 		# remove lines that are unique to ours.
 		orig=`git-unpack-file $2`
 		sz0=`wc -c <"$orig"`
-		diff -u -La/$orig -Lb/$orig $orig $src2 | git apply --no-add
+		$DIFF -u -La/$orig -Lb/$orig $orig $src2 | git apply --no-add
 		sz1=`wc -c <"$orig"`
 
 		# If we do not have enough common material, it is not