portable: typo: libbisd -> libbsd; spotted by naddy@, thanks!

portable: tweak SHA1 detection/libbsd Don't rely on AC_RUN_IFELSE as this breaks cross-compilation (voidlinux). Although this is better replaced with AC_CHECK_DECL. In template/ include checking for libbsd.

portable: template: use as a subproject Move template into its own subproject as it's only needed at compile-time and shouldn't be installed. This needs compiling with the host compiler which is configurable via HOSTCC and HOSTCLFAG environment variables. This diff largely from op@ but additional tweaking from myself.