mesa3d: Pass LDFLAGS to the build config file

Closes #559.

Without the LDFLAGS in the build configuration file
building mesa3d fails with an external toolchain on amd64
machines targeting i686.  The error indicats that libX11
can't be found.

This change passes the --sysroot parameter (along with any
others specified by buildroot for LDFLAGS to the mklib script.

Signed-off-by: Clark Rawlins <clark@bit63.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>