linuxptp: fix libc functionality check in incdefs.sh

The checks in the incdefs.sh script do not fully work when cross
compiling. It probably checks the hosts libc. Just provide the settings
manually, as our musl at least supports all these features.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>

net/linuxptp/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=linuxptp
 PKG_VERSION:=1.8
-PKG_RELEASE:=2
+PKG_RELEASE:=3
 
 PKG_MAINTAINER:=Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tgz
@@ -38,8 +38,10 @@ define Package/linuxptp/description
  computers.
 endef
 
+EXTRA_CFLAGS += -DHAVE_CLOCK_ADJTIME -DHAVE_POSIX_SPAWN -DHAVE_ONESTEP_SYNC
+
 MAKE_VARS += \
-	EXTRA_CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)"
+	EXTRA_CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS) $(EXTRA_CFLAGS)"
 
 define Package/linuxptp/install
 	$(INSTALL_DIR) $(1)/usr/sbin