Fix openssl package finding issue in mac

[zyearn]

No description provided.

[EricPengShuai]

you can also fix the __const__ compile error in gcc version >= 11.2.0 (see apache/brpc#1693)
just set CMAKE_CXX_FLAGS line 52 like this: change -D__const__= to -D__const__=__unused__

set(CMAKE_CXX_FLAGS "${CMAKE_CPP_FLAGS} -DNDEBUG -O2 -D__const__=__unused__ -pipe -W -Wall -Wno-unused-parameter -fPIC -fno-omit-frame-pointer")