Enables -fvisibility=hidden for iOS and Linux, and adds a workaround for GCC 6.3-7.1
Adds a GCC 6 build
Enables diagnostics for C files
Fixes a shadow warning in parsedate.c
We previously used incremental `file(WRITE ...)` commands that gradually recreated the file on every CMake invocation. This sometimes lead to Xcode parsing a partially written file, which in turn breaks building dependend targets. Instead, we're now using a templated configuration file, which ensure that the file is created in one go and hopefully reduces the race condition between CMake and Xcode's automatic project updating.
