Patch for cross-compiling

For cross-compiling of baresip I used the next directory structure:
./re
./rem
./baresip

So, I cross-compiled with next commands in my custom Makefile:
Cross-compile "re"

. some-arm-env-setup; cd re; make SYSROOT=$(TARGET_FSROOT)/usr PREFIX=$(TARGET_FSROOT)/usr/local
Cross-compile "rem"

. some-arm-env-setup; cd rem; make SYSROOT=$(TARGET_FSROOT)/usr PREFIX=$(TARGET_FSROOT)/usr/local LIBRE_INC=../re/include
Cross-compile "baresip"

. some-arm-env-setup; cd baresip; make SYSROOT=$(TARGET_FSROOT)/usr LIBRE_INC=../re/include

In short, I used SYSROOT, PREFIX and LIBRE_INC... And this patch fixes using of SYSROOT for baresip.