Check presence of ar.

This change allows configure to find appropriate ar at cross compile.

configure.ac

@@ -45,6 +45,12 @@ AC_PROG_MKDIR_P
 AC_PROG_RANLIB
 AC_PROG_YACC
 
+AC_CHECK_TOOL([AR], [ar], [:])
+if test "x$AR" = "x:"; then
+  AC_MSG_FAILURE([ar is not found in the system.])
+fi
+AC_SUBST([AR])
+
 AC_PATH_PROG([A2X], [a2x])
 AC_SUBST([A2X])
 AM_CONDITIONAL([HAVE_A2X], [ test "x$A2X" != "x" ])

src/Makefile.am

@@ -553,6 +553,7 @@ if HAVE_KQUEUE
 SRCS += KqueueEventPoll.cc KqueueEventPoll.h
 endif # HAVE_KQUEUE
 
+AR = @AR@
 noinst_LIBRARIES = libaria2c.a
 libaria2c_a_SOURCES = $(SRCS)
 aria2c_LDADD = libaria2c.a @LIBINTL@ @ALLOCA@ @LIBGNUTLS_LIBS@\