Fix symver build error on non-ELF platforms

Fix symver build error on non-ELF platforms

The following error is observed on Microblaze [1] build:

    error: symver is only supported on ELF platforms

due to using __attribute__((symver)) on non-ELF platform.

[1] http://autobuild.buildroot.net/results/1e9/1e965d83d75615f35308440c5db044314a349357/build-end.log

ac_check_attribute_symver.m4 was downloaded from
https://github.com/smuellerDD/libkcapi/blob/master/m4/ac_check_attribute_symver.m4

Fixes: https://github.com/alsa-project/alsa-lib/pull/334
Signed-off-by: Tan En De <ende.tan@starfivetech.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>