liblinear: fix shared library build when -fPIC is required

Some architectures require building with -fPIC for shared libraries. -fPIC is
present in upstream Makefile but we override CFLAGS when running make. Add
-fPIC back when building shared libraries.

Fixes:
http://autobuild.buildroot.net/results/5d6/5d63bf2f1af7977ff81116e767b399d4d0a30a99/
http://autobuild.buildroot.net/results/47d/47d62d66c65cc77da9d48981ca6612257e616a90/
http://autobuild.buildroot.net/results/c81/c816a0982733595438ed7ad90ae1231001c285cc/



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