Improve error message for misconfigured opcache builds

This can be seen when the `./configure` step fails to detect `HAVE_SHM_*`,
e.g. due to missing a necessary dependency to compile the test scripts.
(Run `./configure`, run `yum install libtool-ltdl-devel` for missing dependencies,
then run `make`)

Give a clearer error message than `unknown`
Searching for `opcache "Fatal Error Unable to allocate shared memory segment of"
"unknown: No such file or directory"` reveals issues such as
termux/termux-packages#2234

Author: TysonAndre

ext/opcache/zend_shared_alloc.c

@@ -98,7 +98,7 @@ void zend_shared_alloc_create_lock(char *lockfile_path)
 
 static void no_memory_bailout(size_t allocate_size, char *error)
 {
-	zend_accel_error(ACCEL_LOG_FATAL, "Unable to allocate shared memory segment of %zu bytes: %s: %s (%d)", allocate_size, error?error:"unknown", strerror(errno), errno );
+	zend_accel_error(ACCEL_LOG_FATAL, "Unable to allocate shared memory segment of %zu bytes: %s: %s (%d)", allocate_size, error?error:(handler_table->name?"unknown":"opcache was compiled without shared memory handler implementations"), strerror(errno), errno );
 }
 
 static void copy_shared_segments(void *to, void *from, int count, int size)