make: auto-detect flavour if possible

Makefile

@@ -47,11 +47,15 @@ CORE_HASH=	${CORE_COMMIT:C/^.*-//1}
 
 CORE_ABI?=	16.7
 
-.if "${FLAVOUR}" == OpenSSL || "${FLAVOUR}" == ""
+.if "${FLAVOUR}" == OpenSSL
 CORE_REPOSITORY?=	${CORE_ABI}/latest
 .elif "${FLAVOUR}" == LibreSSL
 CORE_REPOSITORY?=	${CORE_ABI}/libressl
 .else
+. if "${FLAVOUR}" == ""
+_FLAVOUR!=		/usr/local/bin/openssl version
+FLAVOUR=		${_FLAVOUR:[0]}
+. endif
 CORE_REPOSITORY?=	${FLAVOUR}
 .endif