librados/librados_c: check .symver support using cmake

librados/librados_c: check .symver support using cmake

the __asm__(".asmver ..") is a support provided by the compiler, so
would be better to detect it by either checking the compiler identifer
or just try it out.

in this change, instead of checking the building platform, we check this
feature using check_c_source_compiles().

in future, we could support versioned symbols using function attriubte
or symbol tables or version-script.

on platform where symbol versioning is not supported, we might need to
go with a different approach.

Signed-off-by: Kefu Chai <kchai@redhat.com>