Make cmake's check_const_exists() rely on cache

(cherry picked from commit 4c61a04cd5207f2140b65f955559aa4ffc848e9c)

Merge branch 'cmake-configure-fixes-v2'

Fixes in cmake, to make it more like configure and support some
cross-compiling.

* cmake-configure-fixes-v2:
cmake: fix extracting of the version from git

Merge branch 'cmake-configure-fixes-v2'

Fixes in cmake, to make it more like configure and support some
cross-compiling.

* cmake-configure-fixes-v2:
cmake: fix extracting of the version from git (check for number of matches)
Detect arch4random_addrandom() existence
Use off_t instead of ev_off_t for sendfile() (fixes android build)
cmake: detect _GNU_SOURCE not by __GNU_LIBRARY__ only (fallback to _GNU_SOURCE)
Check for WNOWAIT in waitpid() in runtime (not in cmake/configure)
cmake: add <pthread.h> into CMAKE_REQUIRED_INCLUDES for sizeof(pthread_t)
cmake: fix values for #cmakedefine
cmake: drop duplicates from event-config template
cmake: add value for the #cmakedefine macros (like autoconf)
cmake: Fix checking of enum values from sysctl.h

(cherry picked from commit 5aade2d30b6c5eff226cbf7b63fda5a01987ba4f)

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