meson.build: Output DLL names similar to the MSVC projects

This makes the DLL names match those that are produced by the Visual
Studio projects by default.

This, currently, however, names the .lib files same as the ones that
are produced for other platforms (i.e. <libname>-3.lib).  This is
actually not that bad as one can just copy those .lib's into
<libname>-3.0.lib when needed and the binaries that link to those .lib's
ultimately link to the same DLLs, so this should not harm binary
compatibility.

meson.build

@@ -81,7 +81,29 @@ gtk_binary_age = 100 * gtk_minor_version + gtk_micro_version
 cc = meson.get_compiler('c')
 
 if cc.get_id() == 'msvc'
-  gtk_soversion = 'vs@0@'.format(cc.version().split('.')[0])
+  vsver = 0
+  mscver = cc.version().split('.')[0].to_int()
+
+  # pre-Visual Studio 2015 (18.xx.xxxxx or earlier): just subtract 6 from major
+  # version of cc.version() to get the Visual Studio version
+  if mscver < 19
+    vsver = mscver - 6
+  else
+    # Visual Studio 2015 and later (19.xx.xxxxx or later): look at the minor version.
+    # If minor version < 10: Visual Studio 2015,
+    #    10 < minor version < 20: Visual Studio 2017,
+    #    20 < minor version: Visual Studio 2019
+    mscsubver = cc.version().split('.')[1].to_int()
+    if mscsubver < 10
+      vsver = 14
+    elif mscsubver < 20
+      vsver = 15
+    else
+      vsver = 16
+    endif
+  endif
+
+  gtk_soversion = 'vs@0@'.format(vsver)
 else
   gtk_soversion = '0'
 endif