Allow user to specify AR. Derive ar from --host=.

Bug: https://bugs.gentoo.org/730796
--- a/configure.in
+++ b/configure.in
@@ -14,6 +14,7 @@ AC_PROG_INSTALL
 AC_PROG_LN_S
 AC_PROG_MAKE_SET
 AC_PROG_RANLIB
+AC_CHECK_TOOL(AR, ar)
 
 AC_ARG_WITH(xpm-lib,[  --with-xpm-lib          Directory containing libXpm (may be autodetected)],xpm_lib=$withval)
 AC_ARG_WITH(sound,  [  --with-sound            Type of sound to use (native or nas)],sound_type=$withval)
--- a/libsprite/Makefile.in
+++ b/libsprite/Makefile.in
@@ -8,7 +8,7 @@ VPATH = @srcdir@
 .SUFFIXES:
 .SUFFIXES: .c .o
 
-AR = ar
+AR = @AR@
 AR_FLAGS = rc
 RANLIB = @RANLIB@