gdb: specify where to find lzma libraries

gdb: specify where to find lzma libraries

As reported in...

  https://bugs.busybox.net/show_bug.cgi?id=8786

..., gdb fails to build with xz and expat support at the same time when
using an external toolchain.

This is how the problem is shown:

checking for liblzma... no
configure: error: missing liblzma for --with-lzma

And having a deeper look in gdb/config.log we can se a message like this
one:

/lib64/liblzma.so.5: undefined reference to `clock_gettime@GLIBC_2.17'

As Thomas said in the bug report quoted above, that error is related to
the fact that it find the host's lzma.

Adding --with-lzma-prefix=$(STAGING_DIR)/usr to GDB_CONF_OPTS fixes the
problem.

Credit-to: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>