makepkg: Force buildscripts to be in startdir

We expect all source file to lie in $startdir. However, using
"makepkg -p <buildscript>" can currently allows people to specify buildscripts
in other directories. This results in confusion about where other sources
should lie (in startdir or in the directory that the buildscript is in).
Explicitly disable using -p for files in other directories to avoid this issue.

Fixes FS#40293.

Signed-off-by: Allan McRae <allan@archlinux.org>

doc/makepkg.8.txt

@@ -116,7 +116,8 @@ Options
 
 *-p* <buildscript>::
 	Read the package script `buildscript` instead of the `PKGBUILD` default;
-	see linkman:PKGBUILD[5].
+	see linkman:PKGBUILD[5]. The `buildscript` must be located in the directory
+	makepkg is called from.
 
 *-r, \--rmdeps*::
 	Upon successful build, remove any dependencies installed by makepkg

scripts/makepkg.sh.in

@@ -2906,6 +2906,11 @@ else
 		exit 1
 	fi
 
+	if [[ ${BUILDFILE##*/} != "${BUILDFILE}" && ${BUILDFILE} != "${startdir}/${BUILDFILE##*/}" ]]; then
+		error "$(gettext "%s must be in the build directory.")" "$BUILDFILE"
+		exit 1
+	fi
+
 	if [[ ${BUILDFILE:0:1} != "/" ]]; then
 		BUILDFILE="$startdir/$BUILDFILE"
 	fi