dos/meson: add a hint for setting sys_root

I found that cross-compiling with GCC 12.2.0 targeting DJGPP from Linux
on x86_64 that meson used my system include directory
(/usr/include/x86_64-linux-gnu/) and pulled in include files that are
incompatible with DJGPP. Setting sys_root prevents meson from assuming
they're compatible between the build and host systems, fixing those
compile-time errors.

TEST=meson setup --cross-file meson_cross/i586_djgpp_dos.txt; ninja
     libflashrom.h no longer causes "features.h: No such file or
     directory" errors via /usr/include/x86_64-linux-gnu/sys/types.h

Change-Id: Ib9cf70f6f94782c5303fb232aaf4a46192907f66
Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/82212
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>

meson_cross/i586_djgpp_dos.txt

@@ -5,6 +5,11 @@
 # Make sure pkg-config can find your self compiles libpci
 # or add the path of your libpci.pc as 'pkg_config_libdir'
 # under [properies] below.
+#
+# If cross-compiling, you may need to set sys_root in the [properties]
+# section because meson otherwise assumes the same sysroot as the
+# system on which you're building and will get the wrong include files
+# (from /usr/include/x86_64 for example) among other possible issues.
 
 [binaries]
 c     = 'i586-pc-msdosdjgpp-gcc'
@@ -27,3 +32,4 @@ tests = 'disabled'
 ich_descriptors_tool = 'disabled'
 
 [properties]
+sys_root = '/usr/local/djgpp'