[Development] [Android] Use pkg-config to link with external libraries in qmake project

[Oswald Buddenhagen]

On Wed, Dec 20, 2017 at 11:39:20AM +0300, Denis Shienkov wrote:
> This says about the 'sysroot', but, I have no any sysroot for Android kit,
> 
that's just implausible. you must have something that resembles a
sysroot, even if it's not declared as such. if you don't find anything
else, just use the directory that contains the lib and include
directories.

the code is in qt_functions.prf:pkgConfigExecutable(). here you see that
you will need to set %PKG_CONFIG_LIBDIR% and %PKG_CONFIG_SYSROOT_DIR% if
you qt build is not configured for pkg-config use to start with.