PHPC-2715: fix out-of-sources-tree builds

[remicollet]

To reproduce

cd /path/to/src
phpize
mkdir mybuild
cd mybuild
../configure
make

Will output

/dev/shm/BUILD/php-pecl-mongodb2-2.3.1-build/php-pecl-mongodb2-2.3.1/mongo-php-driver-2.3.1/src/libmongocrypt/src/mongocrypt.h:36:8: error: #error No "mongocrypt-config.h" header is available. That file must be generated in order to use libmongocrypt.
   36 |       #error No "mongocrypt-config.h" header is available. That file must \
      |        ^~~~~

[GromNaN]

Thank you @remicollet, for reporting this issue and providing the fix. We will try to add a test job to ensure that there are no regressions in this build environment.