vim: add knob whether elf.h are checked

Previously, it still was checked when there was no elf library in sysroots directory.
Add knob to decide whether elf.h are checked or not.

Fixed by using PACKAGECONFIG to check elf, with default disabled set.

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>