Travis clang does not have the LLVMgold plugin installed.
Disabling LTO is easier than installing the plugin since the CI doesn't
create any artifacts anyway.
Qmake attempts to invoke the linker with the default `g++` binary even
when we changed the binary. This will compile error when using LTO.
For clang:
```
/usr/bin/ld.gold: error: /usr/lib/llvm-3.8/bin/../lib/LLVMgold.so: could
not load plugin library: /usr/lib/llvm-3.8/bin/../lib/LLVMgold.so:
cannot open shared object file: No such file or directory
```
For GCC:
```
lto1: fatal error: bytecode stream generated with LTO version 6.0
instead of the expected 4.1
```
CircleCI Xenial does not come with an up to date version of Clang. It is
therefore required to get the newest version (8). The .pri file has
quickly been edited to be similar to the g++ version check.
The IPolicyConfig file is a third party file and since it describes a
third party API we can't make changes to it. Adding it via the system
includes disables warnings on most compilers.
Encapsulating compiler specific options in a separate file
makes it easier to reason about what is going to happen on
a specific qmake call.
clang-msvc and msvc are separate because they don't support
exactly the same compiler switches.
Vasily