Does not build successfully on OSX El Capitan and earlier due to missing CLOCK_MONOTONIC

[zbentley]

On OSX El Capitan, attempting to build Proxysql 1.4.9 results in failures like this:

mkdir obj
clang -fPIC -c -o obj/c_tokenizer.o c_tokenizer.c -I../include -I../deps/jemalloc/jemalloc/include/jemalloc -I../deps/mariadb-client-library/mariadb_client/include -I../deps/libconfig/libconfig-1.4.9/lib -I../deps/re2/re2 -I../deps/sqlite3/sqlite3 -I../deps/pcre/pcre -I/usr/local/include -I../deps/clickhouse-cpp/clickhouse-cpp -I../deps/libmicrohttpd/libmicrohttpd -I../deps/libmicrohttpd/libmicrohttpd/src/include -I../deps/curl/curl/include -I/usr/local/opt/openssl/include -O2 -ggdb  -Wall -DGITVERSION=\"\" -DNOJEM 
clang++ -fPIC -c -o obj/ProxySQL_GloVars.oo ProxySQL_GloVars.cpp -std=c++11 -I../include -I../deps/jemalloc/jemalloc/include/jemalloc -I../deps/mariadb-client-library/mariadb_client/include -I../deps/libconfig/libconfig-1.4.9/lib -I../deps/re2/re2 -I../deps/sqlite3/sqlite3 -I../deps/pcre/pcre -I/usr/local/include -I../deps/clickhouse-cpp/clickhouse-cpp -I../deps/libmicrohttpd/libmicrohttpd -I../deps/libmicrohttpd/libmicrohttpd/src/include -I../deps/curl/curl/include -I/usr/local/opt/openssl/include -O2 -ggdb  -Wall -DGITVERSION=\"\" -DNOJEM  
clang++ -fPIC -c -o obj/network.oo network.cpp -std=c++11 -I../include -I../deps/jemalloc/jemalloc/include/jemalloc -I../deps/mariadb-client-library/mariadb_client/include -I../deps/libconfig/libconfig-1.4.9/lib -I../deps/re2/re2 -I../deps/sqlite3/sqlite3 -I../deps/pcre/pcre -I/usr/local/include -I../deps/clickhouse-cpp/clickhouse-cpp -I../deps/libmicrohttpd/libmicrohttpd -I../deps/libmicrohttpd/libmicrohttpd/src/include -I../deps/curl/curl/include -I/usr/local/opt/openssl/include -O2 -ggdb  -Wall -DGITVERSION=\"\" -DNOJEM  
clang++ -fPIC -c -o obj/debug.oo debug.cpp -std=c++11 -I../include -I../deps/jemalloc/jemalloc/include/jemalloc -I../deps/mariadb-client-library/mariadb_client/include -I../deps/libconfig/libconfig-1.4.9/lib -I../deps/re2/re2 -I../deps/sqlite3/sqlite3 -I../deps/pcre/pcre -I/usr/local/include -I../deps/clickhouse-cpp/clickhouse-cpp -I../deps/libmicrohttpd/libmicrohttpd -I../deps/libmicrohttpd/libmicrohttpd/src/include -I../deps/curl/curl/include -I/usr/local/opt/openssl/include -O2 -ggdb  -Wall -DGITVERSION=\"\" -DNOJEM  
clang++ -fPIC -c -o obj/configfile.oo configfile.cpp -std=c++11 -I../include -I../deps/jemalloc/jemalloc/include/jemalloc -I../deps/mariadb-client-library/mariadb_client/include -I../deps/libconfig/libconfig-1.4.9/lib -I../deps/re2/re2 -I../deps/sqlite3/sqlite3 -I../deps/pcre/pcre -I/usr/local/include -I../deps/clickhouse-cpp/clickhouse-cpp -I../deps/libmicrohttpd/libmicrohttpd -I../deps/libmicrohttpd/libmicrohttpd/src/include -I../deps/curl/curl/include -I/usr/local/opt/openssl/include -O2 -ggdb  -Wall -DGITVERSION=\"\" -DNOJEM  
debug.cpp:25:16: error: use of undeclared identifier 'CLOCK_MONOTONIC'
        clock_gettime(CLOCK_MONOTONIC, &ts);
                      ^
1 error generated.
make[1]: *** [obj/debug.oo] Error 1

Later versions of OSX build correctly, but not El Cap. Since it's not that old, it would be good to have build support for it.

Since this is debug code, a silly hack is probably possible a la

#ifndef CLOCK_MONOTONIC
#define CLOCK_MONOTONIC SYSTEM_CLOCK
#endif // CLOCK_MONOTONIC

[renecannao]

This approach is already used in gen_utils.h, so it makes sense, see https://github.com/sysown/proxysql/blob/v1.4.10/include/gen_utils.h#L197-L199

[renecannao]

Applied to 1.4.10 and 2.0.0 .

Thanks