[TravisCI] Implement TravisCI build and testing infrastructure for Linux

The Linux builds rely on Docker (using Ubuntu 16.04LTS and Ubuntu
14.04LTS) to build and test Z3 so that builds are easily reproducible.

A build status button has been added to `README.md` so that it is
easy to see the current build status.

More documentation can be found in `contrib/ci/README.md`.

This implementation currently tests 13 different configurations. If
build times become too long we can remove some of them.

Although it would be nice to test macOS builds that requires
significantly more work so I have left this as future work.

contrib/ci/scripts/set_compiler_flags.sh

@@ -0,0 +1,46 @@
+# This script should is intended to be included by other
+# scripts and should not be executed directly
+
+: ${TARGET_ARCH?"TARGET_ARCH must be specified"}
+: ${ASAN_BUILD?"ASAN_BUILD must be specified"}
+: ${UBSAN_BUILD?"UBSAN_BUILD must be specified"}
+: ${CC?"CC must be specified"}
+: ${CXX?"CXX must be specified"}
+
+case ${TARGET_ARCH} in
+  x86_64)
+    CXXFLAGS="${CXXFLAGS} -m64"
+    CFLAGS="${CFLAGS} -m64"
+    ;;
+  i686)
+    CXXFLAGS="${CXXFLAGS} -m32"
+    CFLAGS="${CFLAGS} -m32"
+    ;;
+  *)
+    echo "Unknown arch \"${TARGET_ARCH}\""
+    exit 1
+esac
+
+if [ "X${ASAN_BUILD}" = "X1" ]; then
+  CXXFLAGS="${CXXFLAGS} -fsanitize=address -fno-omit-frame-pointer"
+  CFLAGS="${CFLAGS} -fsanitize=address -fno-omit-frame-pointer"
+fi
+
+if [ "X${UBSAN_BUILD}" = "X1" ]; then
+  CXXFLAGS="${CXXFLAGS} -fsanitize=undefined"
+  CFLAGS="${CFLAGS} -fsanitize=undefined"
+fi
+
+# Report flags
+echo "CXXFLAGS: ${CXXFLAGS}"
+echo "CFLAGS: ${CFLAGS}"
+
+# Report compiler
+echo "CC: ${CC}"
+${CC} --version
+echo "CXX: ${CXX}"
+${CXX} --version
+
+# Export the values
+export CFLAGS
+export CXXFLAGS