main/lua5.1: improve abuild

* Remove _pkgver, it's unnecessary.
* Declare pkgver directly so it can be easily bumped using abump.
* Rename _majorver to _luaver to unify with other lua5.x abuilds.

main/lua5.1/APKBUILD

 # Maintainer: Natanael Copa <ncopa@alpinelinux.org>
 pkgname=lua5.1
 _pkgname=lua
-_majorver=${pkgname#lua}
-pkgver="$_majorver.5"
-_pkgver=$pkgver
+pkgver=5.1.5
+_luaver=${pkgname#lua}
 pkgrel=4
 pkgdesc="A powerful light-weight programming language designed for extending applications."
 url="https://www.lua.org/"
@@ -12,12 +11,12 @@ license="MIT"
 depends_dev="$pkgname"
 makedepends="libtool autoconf automake"
 subpackages="$pkgname-dev $pkgname-doc $pkgname-libs"
-source="https://www.lua.org/ftp/lua-$_pkgver.tar.gz
+source="https://www.lua.org/ftp/lua-$pkgver.tar.gz
 	lua-5.1-make.patch
 	lua-5.1-module_paths.patch
 	lua-5.1-readline.patch
 	"
-builddir="$srcdir/lua-$_pkgver"
+builddir="$srcdir/lua-$pkgver"
 
 prepare() {
 	default_prepare
@@ -68,12 +67,12 @@ package() {
 		alpine_install
 
 	local i; for i in "$pkgdir"/usr/bin/*; do
-		mv "$i" "$i"$_majorver
+		mv "$i" "$i"$_luaver
 	done
 
-	install -D -m 644 etc/lua.pc "$pkgdir"/usr/lib/pkgconfig/lua$_majorver.pc
-	install -D -m 644 doc/lua.1 "$pkgdir"/usr/share/man/man1/lua$_majorver.1
-	install -D -m 644 doc/luac.1 "$pkgdir"/usr/share/man/man1/luac$_majorver.1
+	install -D -m 644 etc/lua.pc "$pkgdir"/usr/lib/pkgconfig/lua$_luaver.pc
+	install -D -m 644 doc/lua.1 "$pkgdir"/usr/share/man/man1/lua$_luaver.1
+	install -D -m 644 doc/luac.1 "$pkgdir"/usr/share/man/man1/luac$_luaver.1
 }
 
 dev() {