makepkg: add tool details to buildinfo to aid determining flags

If a makepkg consumer uses a build wrapper to override compiler
flags this may lead to unreproducible packages as there is no way to
know which exact files were used for tooling that tries to reproduce
said package.

Instead of vendoring the whole used makepkg.conf file into buildinfo,
this patch adds two new properties to the .BUILDINFO file named
BUILDTOOL and BUILDTOOLVER which by default are simply makepkg's own
values. Downstream consumers may override those values: For example in
Arch Linux the devtools package can set those values and allow
reproducible builds tooling to fetch the appropriate makepkg.conf.
Signed-off-by: Levente Polyak <anthraxx@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>

scripts/makepkg.sh.in

@@ -651,6 +651,8 @@ write_buildinfo() {
 	write_kv_pair "builddate" "${SOURCE_DATE_EPOCH}"
 	write_kv_pair "builddir"  "${BUILDDIR}"
 	write_kv_pair "startdir"  "${startdir}"
+	write_kv_pair "buildtool" "${BUILDTOOL:-makepkg}"
+	write_kv_pair "buildtoolver" "${BUILDTOOLVER:-$makepkg_version}"
 	write_kv_pair "buildenv" "${BUILDENV[@]}"
 	write_kv_pair "options" "${OPTIONS[@]}"