gdb: fix target gdb build when host-gawk is built before

When the crosstool-ng backend is used, host-gawk is built as a
dependency of the crosstool-ng package, and therefore an host 'gawk'
binary is installed in $(HOST_DIR).

When the target gdb package is also selected, this unfortunately leads
to a build failure, as reported on
http://buildroot.humanoidz.org/results/f19c0499d08212d8b5100fa9434e1197092957db/build-end.log

.

The problem is that the ./configure of gdb detects gawk in the PATH,
but at compile time, it fails to find gawk. This is due to the fact
that the gdb compilation process is started without the correct path.
This patch fixes this by passing $(TARGET_MAKE_ENV) in the environment
of the gdb compilation process.

A better fix would be to switch gdb to the AUTOTARGETS infrastructure
in the future.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>