Log Message:
Update gnome-common to 3.18.0

- Switch to using XZ-compressed tarballs
- build: Make autoconf-archive support optional
- gnome-autogen: Fix the package name tracing
- macros2: Deprecate GNOME_CXX_WARNINGS
- macros2: Deprecate GNOME_COMPILE_WARNINGS
- gnome-autogen: warn if $srcdir is undefined
- gnome-autogen: warn for deprecated variables
- gnome-autogen: infer PKG_NAME from configure.ac
- README: remove check for automake version in example
- compiler-warnings: Add -Werror=return-type
- macros2: Remove support for gnome-doc-utils
- doc-build: Remove doc-build infrastructure
- macros2: Remove GNOME_COMMON_INIT