package/glibc: use DESTDIR instead of install_root to fix build issue

Building a simple configuration such as:

BR2_aarch64=y
BR2_FORCE_HOST_BUILD=y

Under our reference Docker container, as an out-of-tree build:

./utils/docker-run make O=output-test

will fail during the glibc staging installation step with:

/usr/bin/install -c -m 644 /home/thomas/buildroot/br/output-glibc/build/glibc-2.42-3-gbc13db73937730401d592b33092db6df806d193e/build/libc.a /usr/lib64/libc.a
/usr/bin/install: cannot create regular file '/usr/lib64/libc.a': Permission denied

The problem being that the install_root variable is for some reason
lost along the way. We definitely pass it during the staging
installation step, but then glibc calls a sub-make and in that
submake, install_root is empty.

Observations:

- Building the same configuration inside the Docker container, but
  in-tree, doesn't exhibit the problem.

- Building outside of the Docker container, but with the same make
  version as the one we build due to BR2_FORCE_HOST_BUILD=y doesn't
  exhibit the problem.

However, it turns out that glibc sets install_root to $(DESTDIR). And
passing DESTDIR just works. So we drop our custom
GLIBC_INSTALL_STAGING_OPTS to rely on the default one, and this fixes
the problem. It's not extremely satisfying to not fully understand
what's going on, but after spending quite some time on this, and
having a trivial and actually pretty nice workaround, we simply
propose to use this solution.

There is no autobuilder issue as we're not using our Docker container
to do autobuild builds.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/glibc/glibc.mk

@@ -60,8 +60,6 @@ GLIBC_SUBDIR = build
 
 GLIBC_INSTALL_STAGING = YES
 
-GLIBC_INSTALL_STAGING_OPTS = install_root=$(STAGING_DIR) install
-
 # Thumb build is broken, build in ARM mode
 ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
 GLIBC_EXTRA_CFLAGS += -marm