Support for default configuration location

[szekerest]

When using mapcache in IIS setting IIS environment variables per application is fairly challenging and setting the system wide MAPCACHE_CONFIG_FILE is not always sufficient during deployment. I'd suggest to set up a default configuration location which can be controlled in cmake.

[tbonfort]

+1 if the fallback happens if MAPCACHE_CONFIG_FILE was not found in the environment. I'll let you decide what the default should be on windows. I'd suggest using /etc/mapcache.conf on other platforms.