temp/mesa: fix install prefix, enable release optimizations (!431)

The description below is a quote from Brian Masney in #296.

The pmOS mesa packages need to have -Db_ndebug=true passed to the meson
build so that debug assertions and extra validations are disabled to
improve overall system performance. See [1] for more details. Fedora
also recently made this change: [2]

This change, along with using the master branch of mesa, gets the
Freedreno driver working properly with X11 on the Nexus 5 and presumably
other A3XX-based systems.

--buildtype=release should also be passed to meson for extra
optimizations. See the thread [3] for more details.

[1] https://gitlab.freedesktop.org/mesa/mesa/blob/master/docs/meson.html#L321
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1692426.
[3] https://lists.freedesktop.org/archives/mesa-dev/2019-June/219960.html

[ci:skip-build]: already built successfully in CI

temp/mesa/APKBUILD

@@ -2,7 +2,7 @@
 pkgname=mesa
 pkgver=9999
 _pkgver=19.1.0-rc1
-pkgrel=5
+pkgrel=6
 pkgdesc="[${_pkgver}] Mesa DRI OpenGL library"
 url="https://www.mesa3d.org"
 arch="armhf armv7 aarch64"
@@ -91,8 +91,6 @@ builddir="$srcdir/mesa-$pkgname-$_pkgver"
 
 
 build() {
-	cd "$builddir"
-
 	export CFLAGS="$CFLAGS -D_XOPEN_SOURCE=700"
 	mkdir "$builddir"/build
 	meson -Ddri-search-path="$_dri_driverdir" \
@@ -109,13 +107,14 @@ build() {
 		-Dosmesa=gallium \
 		-Dgbm=true \
 		-Dglx-tls=false \
-		-Dprefix="$pkgdir"/usr "$builddir"/build
+		-Db_ndebug=true \
+		-Dprefix=/usr "$builddir"/build \
+		--buildtype=release
 	ninja -C "$builddir"/build
 }
 
 package() {
-	cd "$builddir"
-	ninja -C "$builddir"/build install
+	DESTDIR="$pkgdir" ninja -C "$builddir"/build install
 }
 
 egl() {