configure.ac: use pkg-config for libelf

configure.ac: use pkg-config for libelf

Use PKG_CHECK_MODULES to get the flags to link libelf

v2: keep AC_CHECK_LIB as a fallback for elfutils provided
libelf that doesn't install a pkg-config file.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>