build-sys: use $XSLTPROC and enforce "unresolved symbols" check
build-sys: use $XSLTPROC and enforce "unresolved symbols" check

trilean/gummiboot,liliang365/my_gummiboot,todorez/tummiboot,FrozenCow/gummiboot,todorez/gummiboot-multiboot2,todorez/tummiboot,miguelinux/gummiboot2systemd-boot,aaronp24/gummiboot,liliang365/my_gummiboot,aaronp24/gummiboot,freedesktop-unofficial-mirror/gummiboot,FrozenCow/gummiboot,todorez/gummiboot-multiboot2,trilean/gummiboot,miguelinux/gummiboot2systemd-boot,freedesktop-unofficial-mirror/gummiboot