Add strtoimax() compatibility function.

Since systems that omit strtoumax() will likely omit strtomax() too, and
likewise for strtoull() and strtoll(), we arrange for the make variables
NO_STRTOUMAX and NO_STRTOULL to cover both the signed and unsigned
functions, and define compatibility implementations for them.

Signed-off-by: Nick Alcock <nix@esperi.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Makefile b/Makefile
index e40ac0c..c5c1fd7 100644
--- a/Makefile
+++ b/Makefile
@@ -57,8 +57,8 @@
 #
 # Define NO_STRLCPY if you don't have strlcpy.
 #
-# Define NO_STRTOUMAX if you don't have strtoumax in the C library.
-# If your compiler also does not support long long or does not have
+# Define NO_STRTOUMAX if you don't have both strtoimax and strtoumax in the
+# C library. If your compiler also does not support long long or does not have
 # strtoull, define NO_STRTOULL.
 #
 # Define NO_SETENV if you don't have setenv in the C library.
@@ -1402,7 +1402,7 @@
 endif
 ifdef NO_STRTOUMAX
 	COMPAT_CFLAGS += -DNO_STRTOUMAX
-	COMPAT_OBJS += compat/strtoumax.o
+	COMPAT_OBJS += compat/strtoumax.o compat/strtoimax.o
 endif
 ifdef NO_STRTOULL
 	COMPAT_CFLAGS += -DNO_STRTOULL