Fix shadowsocks#1659

configure.ac

@@ -81,6 +81,7 @@ AX_TLS([:], [:])
 dnl Checks for crypto libraries
 ss_MBEDTLS
 ss_SODIUM
+ss_CARES
 
 dnl Checks for inet_ntop
 ss_FUNC_INET_NTOP
@@ -218,10 +219,6 @@ AC_CHECK_LIB(socket, connect)
 dnl Checks for library functions.
 AC_CHECK_FUNCS([malloc memset posix_memalign socket])
 
-AC_CHECK_HEADERS([ares.h], [], [AC_MSG_ERROR([Couldn't find libcares. Try installing libc-ares-dev or c-ares-devel.])])
-
-AC_CHECK_LIB([cares], [ares_library_init], [LIBS="-lcares $LIBS"], [AC_MSG_ERROR([Couldn't find libc-ares. Try installing libc-ares-dev or c-ares-devel.])])
-
 AC_CHECK_HEADERS([ev.h libev/ev.h], [], [])
 AC_CHECK_LIB([ev], [ev_loop_destroy], [LIBS="-lev $LIBS"], [AC_MSG_ERROR([Couldn't find libev. Try installing libev-dev@<:@el@:>@.])])
 

m4/cares.m4

@@ -0,0 +1,30 @@
+dnl Check to find the libcares headers/libraries
+
+AC_DEFUN([ss_CARES],
+[
+
+    AC_ARG_WITH(cares,
+    AS_HELP_STRING([--with-cares=DIR], [The c-ares library base directory, or:]),
+    [cares="$withval"
+     CFLAGS="$CFLAGS -I$withval/include"
+     LDFLAGS="$LDFLAGS -L$withval/lib"]
+    )
+
+    AC_ARG_WITH(cares-include,
+    AS_HELP_STRING([--with-cares-include=DIR], [The c-ares library headers directory (without trailing /cares)]),
+    [cares_include="$withval"
+     CFLAGS="$CFLAGS -I$withval"]
+    )
+
+    AC_ARG_WITH(cares-lib,
+    AS_HELP_STRING([--with-cares-lib=DIR], [The c-ares library library directory]),
+    [cares_lib="$withval"
+     LDFLAGS="$LDFLAGS -L$withval"]
+    )
+
+    AC_CHECK_LIB(cares, ares_library_init,
+    [LIBS="-lcares $LIBS"],
+    [AC_MSG_ERROR([The c-ares library libraries not found.])]
+    )
+
+])