vs: Always check VSINSTALLDIR in case VisualStudioVersion is unset

This happened on the CI, so it could happen on people's machines too:

https://ci.appveyor.com/project/jpakkane/meson/build/2870/job/p2n70hg01vp3dkgl
https://ci.appveyor.com/project/jpakkane/meson/build/2870/job/7ifh64mi1999guxt

mesonbuild/backend/vs2010backend.py

@@ -29,26 +29,24 @@ from ..environment import Environment
 
 def autodetect_vs_version(build):
     vs_version = os.getenv('VisualStudioVersion', None)
-    if vs_version:
-        if vs_version == '14.0':
-            from mesonbuild.backend.vs2015backend import Vs2015Backend
-            return Vs2015Backend(build)
-        if vs_version == '15.0':
-            from mesonbuild.backend.vs2017backend import Vs2017Backend
-            return Vs2017Backend(build)
-        raise MesonException('Could not detect Visual Studio (unknown Visual Studio version: "{}")!\n'
-                             'Please specify the exact backend to use.'.format(vs_version))
-
     vs_install_dir = os.getenv('VSINSTALLDIR', None)
-    if not vs_install_dir:
-        raise MesonException('Could not detect Visual Studio (neither VisualStudioVersion nor VSINSTALLDIR set in '
-                             'environment)!\nPlease specify the exact backend to use.')
-
+    if not vs_version and not vs_install_dir:
+        raise MesonException('Could not detect Visual Studio: VisualStudioVersion and VSINSTALLDIR are unset!\n'
+                             'Are we inside a Visual Studio build environment? '
+                             'You can also try specifying the exact backend to use.')
+    # VisualStudioVersion is set since Visual Studio 12.0, but sometimes
+    # vcvarsall.bat doesn't set it, so also use VSINSTALLDIR
+    if vs_version == '14.0' or 'Visual Studio 14' in vs_install_dir:
+        from mesonbuild.backend.vs2015backend import Vs2015Backend
+        return Vs2015Backend(build)
+    if vs_version == '15.0' or 'Visual Studio 17' in vs_install_dir or \
+       'Visual Studio\\2017' in vs_install_dir:
+        from mesonbuild.backend.vs2017backend import Vs2017Backend
+        return Vs2017Backend(build)
     if 'Visual Studio 10.0' in vs_install_dir:
         return Vs2010Backend(build)
-
-    raise MesonException('Could not detect Visual Studio (unknown VSINSTALLDIR: "{}")!\n'
-                         'Please specify the exact backend to use.'.format(vs_install_dir))
+    raise MesonException('Could not detect Visual Studio using VisualStudioVersion: {!r} or VSINSTALLDIR: {!r}!\n'
+                         'Please specify the exact backend to use.'.format(vs_version, vs_install_dir))
 
 def split_o_flags_args(args):
     """