use system headers for system libnghttp2

[remicollet]

Affects 5.0.3, removing thirdparty/nghttp2 (to ensure bundled copy is not used) and using --with-nghttp2-dir=/usr

In file included from /dev/shm/BUILD/php-pecl-swoole5-5.0.3/NTS/ext-src/swoole_admin_server.cc:19:
/dev/shm/BUILD/php-pecl-swoole5-5.0.3/NTS/ext-src/php_swoole_http.h:37:10: fatal error: thirdparty/nghttp2/nghttp2.h: No such file or directory
   37 | #include "thirdparty/nghttp2/nghttp2.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

This may raise strange issue using bundled headers with system library (API may differ)

[codecov]

Codecov Report

Merging #5038 (2714169) into master (0d83f46) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5038      +/-   ##
==========================================
+ Coverage   73.11%   73.13%   +0.02%     
==========================================
  Files          69       69              
  Lines       14793    14793              
==========================================
+ Hits        10816    10819       +3     
+ Misses       3977     3974       -3     

see 4 files with indirect coverage changes

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