autotools: Take VPATH builds into account when generating testvectors

Author: real-or-random

Makefile.am

@@ -227,7 +227,7 @@ TESTVECTORS = src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.h
 
 src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.h:
 	mkdir -p $(@D)
-	python3 tools/tests_wycheproof_generate.py src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.json > $@
+	python3 $(top_srcdir)/tools/tests_wycheproof_generate.py $(top_srcdir)/src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.json > $@
 
 testvectors: $(TESTVECTORS)