lz4: improve static only build support

The current method of supporting static only build, removal of all lines
that match the SHARED regex from lib/Makefile, is crude and fragile.
Instead, patch lib/Makefile to allow disable of shared libraries.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>