Add Windows to TravisCI

Implement the generation of Travis jobs for Windows. Currently, the
generated jobs replicate Appveyor setup and complete successfully. There
is support for MinGW GCC and MSVC compilers as well as 64 and 32 bit
compilation. Linux and MacOS jobs behave identically, but some
environment variables change - CROSS_COMPILE_32BIT=yes is added for
builds with cross compilation, empty COMPILER_FLAGS are not set anymore.

scripts/linux/before_install.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+set -ev
+
+if [[ "$TRAVIS_OS_NAME" != "linux" ]]; then
+    echo "Incorrect \$TRAVIS_OS_NAME: expected linux, got $TRAVIS_OS_NAME"
+    exit 1
+fi
+
+if [[ "$CROSS_COMPILE_32BIT" == "yes" ]]; then
+    sudo apt-get update
+    sudo apt-get -y install gcc-multilib g++-multilib
+fi