autotools: Make Makefile.am.libasncodec more autotools friendly
diff --git a/libasn1compiler/asn1c_save.c b/libasn1compiler/asn1c_save.c
index 47888d0..1cc11a2 100644
--- a/libasn1compiler/asn1c_save.c
+++ b/libasn1compiler/asn1c_save.c
@@ -147,10 +147,12 @@
 	safe_fprintf(
 		mkf,
 		"\n\n"
-		"lib_LTLIBRARIES=libasncodec.la\n"
+		"lib_LTLIBRARIES+=libasncodec.la\n"
 		"libasncodec_la_SOURCES="
 		"$(ASN_MODULE_SRCS) $(ASN_MODULE_HDRS)\n"
-		"libasncodec_la_CFLAGS=$(ASN_MODULE_CFLAGS)\n");
+		"libasncodec_la_CPPFLAGS=-I$(top_srcdir)/%s\n"
+		"libasncodec_la_CFLAGS=$(ASN_MODULE_CFLAGS)\n"
+		"libasncodec_la_LDFLAGS=-lm\n", destdir);
 	fclose(mkf);
 	safe_fprintf(stderr, "Generated %s%s\n", destdir, makefile_name);