As reported in this community forum post, the libmongocrypt pkg-config script has a broken dependency. Further investigation by wan.bachtiar revealed that this problem affects libmongocrypt 1.2.1 but does not exist for 1.0.4.

The broken dependency needs to be corrected, and the build updated to include a pkg-config invocation like this to ensure that scripts that are shipped function correctly;

$ pkg-config --print-errors --exists libmongocrypt || echo $?
Package libbson-static-1.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libbson-static-1.0.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libbson-static-1.0', required by 'libmongocrypt', not found
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