The decimal128 support implemented in MONGOCRYPT-483 introduced a dependency on Intel DFP. The initial implementation downloads the Intel DFP library source distribution from the Internet for each build. it is necessary to support disconnected builds (i.e., builds without network access), which entails the following improvements:

• include the the Intel DFP sources as a vendored third-party module of libmongocrypt (similar to how zlib is handled for the C driver)
• add the option for the user to build using a system-installed Intel DFP library rather than using the vendored Intel DFP sources