make: Avoid using host include for cross-compiling

Prepend include paths with the toolchain's sysroot directory.
In case of a non-sysroot-aware toolchain, this does not help,
but does not break either.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

pppd/Makefile.linux

@@ -125,7 +125,7 @@ CFLAGS   += -DHAS_SHADOW
 #LIBS     += -lshadow $(LIBS)
 endif
 
-ifneq ($(wildcard /usr/include/crypt.h),)
+ifneq ($(wildcard $(shell $CC --print-sysroot)/usr/include/crypt.h),)
 CFLAGS  += -DHAVE_CRYPT_H=1
 LIBS	+= -lcrypt
 endif
@@ -137,7 +137,7 @@ endif
 
 ifdef NEEDDES
 ifndef USE_CRYPT
-CFLAGS   += -I/usr/include/openssl
+CFLAGS   += -I$(shell $CC --print-sysroot)/usr/include/openssl
 LIBS     += -lcrypto
 else
 CFLAGS   += -DUSE_CRYPT=1