tools: fix portability issue

Check if the compiler defines __linux__, instead of assuming that the
host OS is the same as the target OS.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
--- a/src/tools/Makefile
+++ b/src/tools/Makefile
@@ -43,7 +43,7 @@ CFLAGS += -DRUNSTATEDIR="\"$(RUNSTATEDIR
 ifeq ($(DEBUG_TOOLS),y)
 CFLAGS += -g
 endif
-ifeq ($(shell uname -s),Linux)
+ifeq ($(strip $(shell echo __linux__ | $(CC) -E - | grep -v '^\#')),1)
 LIBMNL_CFLAGS := $(shell $(PKG_CONFIG) --cflags libmnl 2>/dev/null)
 LIBMNL_LDLIBS := $(shell $(PKG_CONFIG) --libs libmnl 2>/dev/null || echo -lmnl)
 CFLAGS += $(LIBMNL_CFLAGS)