selinux: use grep to check api supports (#402)

This checks `selinux_state` and `current_sid` supports in a raw way.
Feels more reliable than the version checks.

Supersedes #401, fixes #280, fixes #400.

kernel/selinux/Makefile

@@ -2,6 +2,13 @@ obj-y += selinux.o
 obj-y += sepolicy.o
 obj-y += rules.o
 
+ifeq ($(shell grep -q " current_sid(void)" $(srctree)/security/selinux/include/objsec.h; echo $$?),0)
+ccflags-y += -DKSU_COMPAT_HAS_CURRENT_SID
+endif
+
+ifeq ($(shell grep -q "struct selinux_state " $(srctree)/security/selinux/include/security.h; echo $$?),0)
+ccflags-y += -DKSU_COMPAT_HAS_SELINUX_STATE
+endif
 
 ccflags-y += -Wno-implicit-function-declaration -Wno-strict-prototypes -Wno-int-conversion
 ccflags-y += -Wno-macro-redefined -Wno-declaration-after-statement -Wno-unused-function