Add configure conditional to control validation

Allow disabling of AppData XML validation during the check phase by
passing --disable-appdata-validate to configure. Only force validation
if --enable-appdata-validate is passed to configure. The default is to
validate AppData XML only if appdata-validate is installed.

https://github.com/hughsie/appdata-tools/issues/15
Merge pull request #17 from hughsie/issue-15

Add configure conditional to control validation