lalapps: re-add liblalapps to BUILT_SOURCES, prevent build on 'make dist'

Description

• Fixes commit 7b026038 from !1025 (merged)
• BUILT_SOURCES seems to be the only way to get liblalapps built before everything else; making it a dependency of all, install, etc. doesn't work
• To fix problem with 'make dist' trying to build liblalapps, use presence of $(distdir) to avoid building liblalapps when building a distribution
• Remove setting LDADD in configure.ac since it is already set in lalapps.am
• Fix up Makefile.am to set BUILT_SOURCES first before sourcing lalapps.am

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

Review Status

@adam-mercer to review