feature-test: add CONFIG_COMPAT compile option

I'll wrap all compatible code in this CONFIG_COMPAT define.
As I'll wrap also compatible parasite generation in this,
it's also makefile variable, rather than just C define.
The test itself consists of including stubs-32.h, which is
glibc6-i686 presence test and is compiled with -m32 option,
which is test for gcc-multilib.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Makefile.config

@@ -19,6 +19,14 @@ CONFIG_FILE = $(SRC_DIR)/.config
 $(CONFIG_FILE):
 	touch $(CONFIG_FILE)
 
+ifeq ($(SRCARCH),x86)
+# CONFIG_COMPAT is only for x86 now, no need for compile-test other archs
+ifeq ($(call try-cc,$(FEATURE_TEST_X86_COMPAT),-m32),true)
+        export CONFIG_COMPAT := y
+        FEATURE_DEFINES	+= -DCONFIG_COMPAT
+endif
+endif
+
 export DEFINES += $(FEATURE_DEFINES)
 export CFLAGS += $(FEATURE_DEFINES)
 

scripts/feature-tests.mak

@@ -88,3 +88,15 @@ int main(int argc, char *argv[], char *envp[])
 }
 
 endef
+
+define FEATURE_TEST_X86_COMPAT
+
+/* Test for glibc-devel.i686 presence */
+#include <gnu/stubs-32.h>
+
+int main(int argc, char **argv)
+{
+        return 0;
+}
+
+endef