configure: error out on unsupported MSVC versions

The FATE tests for MSVC versions older than 2013 are untested in FATE
and apparently are no longer supported.

This commit makes the configure process error out in case an older version
is used, and suggests to use a supported version of MSVC to compile.

This also changes the documentation to reflect this.

As discussed on IRC:

2018-05-12 19:45:16     jamrial then again, most of those were for old msvc, and i think we're not supporting versions older than 2013 (first one c99 compliant) anymore
2018-05-12 19:45:43     +JEEB   yea, I think 2013 update 2 is needed

22:53 <@atomnuker> nevcairiel: which commit broke/unsupported support for msvc 2013?
23:23 <@atomnuker> okay, it was JEEB
23:25 <+JEEB> which was for 2012 and older
23:25 <+JEEB> and IIRC we no longer test those in FATE so that was my assumption
23:26 <+JEEB> 2013 is when MS got trolled enough to actually update their C part
23:26 <+JEEB> aand actually advertised FFmpeg support
23:26 <+JEEB> (although it was semi-failing until VS2013 update 1 or 2)

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

configure

@@ -3988,8 +3988,7 @@ case "$toolchain" in
             cc_default="cl"
             cxx_default="cl"
         else
-            cc_default="c99wrap cl"
-            cxx_default="c99wrap cl"
+            die "Unsupported MSVC version (2013 or newer required)"
         fi
         ld_default="$source_path/compat/windows/mslink"
         nm_default="dumpbin -symbols"