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>
---
--- src/tools/Makefile
+++ src/tools/Makefile
@@ -35,8 +35,6 @@
 endif
 endif
 
-PLATFORM ?= $(shell uname -s | tr '[:upper:]' '[:lower:]')
-
 CFLAGS ?= -O3
 CFLAGS += -std=gnu99 -D_GNU_SOURCE
 CFLAGS += -Wall -Wextra
@@ -45,7 +43,7 @@
 ifeq ($(DEBUG_TOOLS),y)
 CFLAGS += -g
 endif
-ifeq ($(PLATFORM),linux)
+ifeq ($(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)