gdb: fix build of host-gdb 7.4

The build of host-gdb 7.4 fails due to some texinfo issue. To avoid
that, use the same trick as is used for the target variant of gdb:
tell gdb that makeinfo is missing.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>