Add .travis.yml

For now this is mainly useful if you work on a fork of the libxml2
mirror on GitHub:

    https://github.com/GNOME/libxml2

Start with two build setups:

- GCC with as many GNU extensions disabled as possible, trying to
  emulate a C89 compiler on a POSIX system.

- clang with ASan and UBSan.

The Python tests don't set an exit code, so Travis won't detect
failures. The same goes for "make tests", but we only run "make check"
anyway.

.travis.yml

@@ -0,0 +1,22 @@
+language: c
+sudo: false
+addons:
+  apt:
+    packages:
+      # Some tests require the DTDs.
+      w3c-sgml-lib
+matrix:
+  include:
+    # Try to emulate a C89 compiler on a POSIX system by disabling as many
+    # GNU extensions as possible. -Dlinux is required to make the weak
+    # pthread symbols work.
+    - compiler: gcc
+      env: CFLAGS="-O2 -std=c89 -D_POSIX_C_SOURCE=200809L -Dlinux"
+    # clang with AddressSanitizer and UndefinedBehaviorSanitizer.
+    - compiler: clang
+      dist: trusty
+      env: CONFIG="--without-python"
+           CFLAGS="-O2 -g -fno-omit-frame-pointer -fsanitize=address,undefined -fno-sanitize-recover"
+script: sh autogen.sh $CONFIG && make -j2 V=1 && make check
+git:
+  depth: 10