Can't build a 32-bit GTK2 app on a 64-bit system

[Iouri Kloubakov]

Description: Ubuntu 16.04.1 LTS
Release: 16.04

gcc-multilib:
  Installed: 4:5.3.1-1ubuntu1
  Candidate: 4:5.3.1-1ubuntu1

apt install build-essential gcc-multilib libgtk2.0-dev

The build fails because there is only 64-bit version of glibconfig.h is installed:
/usr/lib/x86_64-linux-gnu/glib-2.0/include/glibconfig.h

Running gcc -m32 gives an error:
/usr/include/glib-2.0/glib/gtypes.h:422:3: error: '_GStaticAssertCompileTimeAssertion_0' declared as an array with a negative size
G_STATIC_ASSERT(sizeof (unsigned long long) == sizeof (guint64));
because guint64 is defined as (unsigned long) on a 64-bit platform.

Also, there is no 32-bit version of the "pkgconfig" files, so you can't use:
$(pkg-config --cflags gtk+-2.0) with, for example, PKG_CONFIG_PATH=/usr/lib32/pkgconfig.

[Matthias Klose (doko)]

you should install crossbuild-essential-i386 and try to build with dpkg-buildpackage -a i386