configure.ac: check validity of sys/shm.h

Some version of Android < 26 have sys/shm.h but it doesn't contain the
functions that shall be exposed. So as to avoid misconfiguration, only
define HAVE_SYS_SHM_H if the header exists AND that it exposes the
required functions.
13 jobs for !10 with android/fix-shm-detection/1 in 139 minutes and 34 seconds (queued for 2 seconds)
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