MINIFICPP-109: Add header guards for civetweb so users who don't have pragma defs can build.
MINIFICPP-109: Add header guards for civetweb so users who don't have pragma defs can build.

Pulled from merged PR located at :
https://github.com/KevinCalderone/civetweb/commit/1b25c739802bac622edc83f9ca9f0f9e695879a4

This closes #138.

Signed-off-by: Aldrin Piri <8746015474cd01110181ed51735247560fd08de4@apache.org>

benqiu2016/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,apiri/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,benqiu2016/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,apiri/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,apiri/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,apiri/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,apiri/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,benqiu2016/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,achristianson/nifi-minifi-cpp