Fix undefined reference with old versions of openssl
Fix undefined reference with old versions of openssl

Versions prior to 0.9.8f  did not have this function, rhel/centos5 are still on a
heavily backported version of 0.9.8e and theoretically supported until March 2017

Without this ifdef, I get the following link failure:
```
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `openssl_connect':
openssl_stream.c:(.text+0x45a): undefined reference to `SSL_set_tlsext_host_name'
collect2: error: ld returned 1 exit status
make[6]: *** [libgit2_clar] Error 1
```
mingyaaaa/libgit2,JIghtuse/libgit2,linquize/libgit2,linquize/libgit2,yosefhackmon/libgit2,mhp/libgit2,leoyanggit/libgit2,yongthecoder/libgit2,KTXSoftware/libgit2,leoyanggit/libgit2,linquize/libgit2,mingyaaaa/libgit2,mingyaaaa/libgit2,MrHacky/libgit2,stewid/libgit2,since2014/libgit2,mcanthony/libgit2,claudelee/libgit2,mcanthony/libgit2,claudelee/libgit2,iankronquist/libgit2,sim0629/libgit2,joshtriplett/libgit2,dleehr/libgit2,sygool/libgit2,skabel/manguse,oaastest/libgit2,dleehr/libgit2,ardumont/libgit2,stewid/libgit2,sim0629/libgit2,t0xicCode/libgit2,kenprice/libgit2,magnus98/TEST,KTXSoftware/libgit2,oaastest/libgit2,falqas/libgit2,claudelee/libgit2,oaastest/libgit2,since2014/libgit2,saurabhsuniljain/libgit2,saurabhsuniljain/libgit2,spraints/libgit2,yongthecoder/libgit2,mhp/libgit2,skabel/manguse,kenprice/libgit2,stewid/libgit2,mcanthony/libgit2,falqas/libgit2,skabel/manguse,mingyaaaa/libgit2,jeffhostetler/public_libgit2,spraints/libgit2,Corillian/libgit2,Corillian/libgit2,iankronquist/libgit2,dleehr/libgit2,sim0629/libgit2,jeffhostetler/public_libgit2,mingyaaaa/libgit2,joshtriplett/libgit2,ardumont/libgit2,ardumont/libgit2,Tousiph/Demo1,oaastest/libgit2,spraints/libgit2,yosefhackmon/libgit2,JIghtuse/libgit2,mrksrm/Mingijura,falqas/libgit2,oaastest/libgit2,oaastest/libgit2,saurabhsuniljain/libgit2,yongthecoder/libgit2,sygool/libgit2,yosefhackmon/libgit2,magnus98/TEST,kenprice/libgit2,JIghtuse/libgit2,jeffhostetler/public_libgit2,t0xicCode/libgit2,t0xicCode/libgit2,sygool/libgit2,jeffhostetler/public_libgit2,claudelee/libgit2,JIghtuse/libgit2,linquize/libgit2,t0xicCode/libgit2,saurabhsuniljain/libgit2,Tousiph/Demo1,yongthecoder/libgit2,joshtriplett/libgit2,leoyanggit/libgit2,skabel/manguse,mrksrm/Mingijura,mcanthony/libgit2,falqas/libgit2,linquize/libgit2,KTXSoftware/libgit2,Corillian/libgit2,since2014/libgit2,mcanthony/libgit2,magnus98/TEST,mrksrm/Mingijura,saurabhsuniljain/libgit2,iankronquist/libgit2,leoyanggit/libgit2,t0xicCode/libgit2,Tousiph/Demo1,t0xicCode/libgit2,Tousiph/Demo1,linquize/libgit2,spraints/libgit2,claudelee/libgit2,MrHacky/libgit2,ardumont/libgit2,mrksrm/Mingijura,mcanthony/libgit2,Tousiph/Demo1,Corillian/libgit2,ardumont/libgit2,leoyanggit/libgit2,since2014/libgit2,JIghtuse/libgit2,falqas/libgit2,sim0629/libgit2,mrksrm/Mingijura,MrHacky/libgit2,stewid/libgit2,mhp/libgit2,KTXSoftware/libgit2,joshtriplett/libgit2,mhp/libgit2,magnus98/TEST,yosefhackmon/libgit2,magnus98/TEST,stewid/libgit2,sygool/libgit2,kissthink/libgit2,jeffhostetler/public_libgit2,mrksrm/Mingijura,leoyanggit/libgit2,since2014/libgit2,dleehr/libgit2,spraints/libgit2,joshtriplett/libgit2,joshtriplett/libgit2,saurabhsuniljain/libgit2,yongthecoder/libgit2,Corillian/libgit2,iankronquist/libgit2,mhp/libgit2,KTXSoftware/libgit2,sim0629/libgit2,claudelee/libgit2,spraints/libgit2,kissthink/libgit2,sygool/libgit2,dleehr/libgit2,mhp/libgit2,since2014/libgit2,yosefhackmon/libgit2,Corillian/libgit2,falqas/libgit2,magnus98/TEST,kenprice/libgit2,JIghtuse/libgit2,jeffhostetler/public_libgit2,iankronquist/libgit2,kissthink/libgit2,Tousiph/Demo1,dleehr/libgit2,yongthecoder/libgit2,ardumont/libgit2,MrHacky/libgit2,MrHacky/libgit2,kissthink/libgit2,MrHacky/libgit2,mingyaaaa/libgit2,yosefhackmon/libgit2,sygool/libgit2,skabel/manguse,sim0629/libgit2,kissthink/libgit2,skabel/manguse,iankronquist/libgit2,KTXSoftware/libgit2,kenprice/libgit2,kissthink/libgit2,stewid/libgit2,kenprice/libgit2