In mongodb/mongo-php-driver#756, a user reported a failure when libmongoc attempted to include sals/sash.h, despite the fact that SASL was detected during configure.

It appears this is because we never appended "/include" to the include path, dating back to 1.0.0. Contrast our PHP_ADD_INCLUDE line with other extensions, such as sodium, which do add the "/include" suffix after they've searched for the header. Independent of this, PHP_CHECK_LIBRARY (which wraps AC_CHECK_LIB) finds the library just fine.

I expect this went unreported for so long because usr/include and usr/local/include are part of GCC's standard include paths for most of our users' platforms. The fellow in the ticket above was using FreeBSD.