Improve support for <MODULE>_PATH options

Several modules, including DBus, MySQL, and OpenSSL have
configure options of the form <MODULE>_PATH, which is used
on Windows (where pkg-config is not present) to specify the
locations of third-party libraries.  These switches had been
implemented by adding extra variables which were referenced
in .pro files, to add the appropriate compiler and linker
switches.  This is undesirable because it means there are
two independent paths for adding the switches to the build,
which can get out of sync with each other, and indeed this
had happened for some of the DBus tools.

To remedy the situation, all three of the switches were
reworked so that they added values directly to the principal
variables that are used in the project files.  This reduces
maintenance, by ensuring that the pkg-config and non-pkg-config
paths appear the same to the rest of the build system.

Change-Id: Iae342f1d14b79fbcfef9fe38aadc803ad3141799
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>