[MSVC 14.0] jpeg-turbo breakage on master

@Stephan,

could you please have a look, why this change: [1] broke jpeg-turbo 
compilation on MSVC 14.0.

For the record, MS did a great job in splitting the include path in two

* compiler internal
* UCRT (Universal C Runtime)

This is mapped in configure.ac: [2] and respected in external modules.
When you overwrite CFLAGS in the invocation of gcc-warpper, then
presumably the UCRT include path is lost.

Here is the config.log with offended commit: [3]. And this one after
I've reverted it: [4], you see the include, it worked in the end.
This solved the breakage, caused by: [1].

[1] https://cgit.freedesktop.org/libreoffice/core/commit/?id=8729504f93
c8b59cb2fd6145a1f02987d5fcdb47
[2] http://paste.openstack.org/show/487351
[3] http://paste.openstack.org/show/487348
[4] http://paste.openstack.org/show/487349