This is necessary to support Decimal128 BSON types in range indexes (PHPC-2197).

MONGOCRYPT-483 introduced Intel DFP as an optional dependency in libmongocrypt 1.7.0. Subsequently, MONGOCRYPT-517 vendored the library to allow offline builds (1.7.1). Note that libmongocrypt also applies patches to the vendored source, which complicates things a bit.

Supporting this will require PHPC to port existing logic from libmongocrypt's CMake scripts to Autotools:

• cmake/ImportDFP.cmake is responsible for determining whether to link or build the vendored source.
• cmake/IntelDFP.cmake is responsible for building the vendored source.

Note that pkg-config cannot be used to detect and link IntelDFP (see: mongodb/libmongocrypt#550).