depends: give miniupnpc cppflags
depends: make LDFLAGS act like the other flags
depends: add docs for debug
depends: disable reduced exports for debug builds

Some debug options may not be compatible.
tests: don't split an empty string
script: don't read past the end
travis: use debug for one build

For the all-off build, enable the wallet and debug. This ensures that debug
options will catch wallet problems as well.

In order to make sure the no-wallet path is still tested, disable the wallet
in the other x86_64 build.