apr: Use correct strerror_r implementation based on libc type

musl does not implement GNU extention of strerror_r but XSI compliant
version, therefore add it via a packageconfig to set right variables
during configure to cache the value.

configure detection logic depends on runtime test which will always be
wrong on cross compiles therefore backport a patch to make it possible
to cache the needed configure variable.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>