src/poky:man-db: fix broken requirement for flex

Normally flex-native in the sysroot via the toolchain, but different
toolchains may not depend on flex-native (eg, external-arm-toolchain).

This results in a configure error:

  checking for flex... no
  configure: error: flex is required when building from revision control

Now we're not building from revision control, but the configure script
is broken with out-of-tree builds and checks the (empty) build tree for
pre-generated sources.  Apply a fix to look in the source tree instead.

(From OE-Core rev: 544d8ee19b5ac74a841722a3e000019d2e6ab4f8)

Signed-off-by: Ross Burton <ross.burtonarm.com>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>
Signed-off-by: Ross Burton <ross.burtonarm.com>

Author: rossburton