VOMS Server, C/C++ APIs and clients v2.1.0-rc3

Main changes

• Allow to skip OpenSSL initialization by @giacomini in #94
• Initialize OpenSSL only below version 1.1 by @giacomini in #95
• Don't use macros in AC_CHECK_LIB by @ellert in #58
• Change default proxy cert key length to 2048 bits by @ellert in #75
• gcc7 by @ellert in #56
• GSOAP checks are only needed for server. by @msalle in #74
• Remove globus version detection by @ellert in #97
• OpenSSL-3.0.0-beta1 fixes by @msalle in #98
• Declare fields of AC_TARGET as optional by @giacomini in #103
• Port to centos9 (stream) by @giacomini in #101

Full Changelog: v2.1.0-rc2...v2.1.0-rc3