Change libdl and libz checks to warnings when not found.
diff --git a/configure.in b/configure.in
index 444528c..232483d 100644
--- a/configure.in
+++ b/configure.in
@@ -143,9 +143,9 @@
    CFLAGS="$CFLAGS $(pkg-config --cflags openssl)";
 
    AC_CHECK_LIB([dl], [dlopen], [],
-             [AC_MSG_FAILURE([can't find libdl])])
+             [AC_MSG_WARN([can't find libdl])])
    AC_CHECK_LIB([z], [inflate], [],
-             [AC_MSG_FAILURE([can't find libz])])
+             [AC_MSG_WARN([can't find libz])])
    AC_CHECK_LIB([crypto], [EVP_EncryptInit], [],
              [AC_MSG_FAILURE([can't find openssl >1.0.1 crypto lib])])
    AC_CHECK_LIB([crypto], [EVP_aes_128_ctr], [],