Dune 1.7.2

Add ${corrected-suffix}, ${library-name} and a few other variables to the list of variables to upgrade. This fixes the support for various framework producing corrections (#1840, #1853, @diml)

Fix $ dune subst failing because the build directory wasn't set. (#1854, fix #1846, @rgrinberg)

Configurator: Add warning to Pkg_config.query when a full package expression is used. Add Pkg_config.query_expr for cases when the full power of pkg-config's querying is needed (#1842, fix #1833, @rgrinberg)

Fix unavailable, optional implementations eagerly breaking the build (#1857, fix #1856, @rgrinberg)