Add Visual Studio 2012/2013 backends (#8803)

* backends: Add a Visual Studio 2013 backend

This is more-or-less a quick port from the VS2015 backend, except that
we update the Visual Studio version strings and toolset versions
accordingly.  Also correct the generator string for Visual Studio 2015
in mesonbuild/cmake/common.py.

* backend: Add VS2012 backend

Similar to what we did for Visual Studio 2013, add a Visual Studio 2012
backend.

* vs2010backend.py: Implement `link_whole:` if needed

We actually need Visual Studio 2015 Update 2 to use `/WHOLEARCHIVE:`,
which is what we are currently using for `link_whole:` on Visual Studio.
For Visual Studio versions before that, we need to expand from the
static targets that were indicated by `link_whole:`, and any of the
sub-dependent targets that were pulled in via the dependent target's
`link_whole:`.  This wil ensure `link_whole:` would actually work in
such cases.

* vs2010backend.py: Handle objects from generated sources

Unforunately, we can't use backends.determine_ext_objs() reliably, as
the Visual Studio backends handle this differently.

* vs2010backend.py: Fix generating VS2010 projects

Visual Studio 2010 (at least the Express Edition) does not set the envvar
%VisualStudioVersion% in its command prompt, so fix generating VS2010
projects by taking account into this, so that we can determine the location
of vcvarsall.bat correctly.

* whole archive test: Disable on vs2012/2013 backends too

The Visual Studio 2012/2013 IDE has problems handling the items that would be
generated from this test case, so skip this test when using
--backend=vs[2012|2013].  This test does work for the Ninja backend when
VS2012 or VS2013 is used, though.

Consolidate this error message with XCode along with the vs2010 backend.

* docs: Add the new vs2012 and vs2013 backends

Let people know that we have backends for vs2012 and 2013.  Also let
people know that generating Visual Studio 2010 projects have been fixed
and the pre-vs2015 backends now handle the `link_whole:` project option.

3 files changed, 18 insertions, 3 deletions

diff --git a/docs/markdown/Builtin-options.md b/docs/markdown/Builtin-options.md
index ce33fce..0536e77 100644
--- a/docs/markdown/Builtin-options.md
+++ b/docs/markdown/Builtin-options.md
@@ -69,7 +69,7 @@ machine](#specifying-options-per-machine) section for details.
 | Option                               | Default value | Description                                                    | Is per machine | Is per subproject |
 | ------                               | ------------- | -----------                                                    | -------------- | ----------------- |
 | auto_features {enabled, disabled, auto} | auto       | Override value of all 'auto' features                          | no             | no                |
-| backend {ninja, vs,<br>vs2010, vs2015, vs2017, vs2019, xcode} | ninja | Backend to use                                | no             | no                |
+| backend {ninja, vs,<br>vs2010, vs2012, vs2013, vs2015, vs2017, vs2019, xcode} | ninja | Backend to use                | no             | no                |
 | buildtype {plain, debug,<br>debugoptimized, release, minsize, custom} | debug |  Build type to use                    | no             | no                |
 | debug                                | true          | Debug                                                          | no             | no                |
 | default_library {shared, static, both} | shared      | Default library type                                           | no             | yes               |
diff --git a/docs/markdown/Reference-manual.md b/docs/markdown/Reference-manual.md
index de7f9f2..388c975 100644
--- a/docs/markdown/Reference-manual.md
+++ b/docs/markdown/Reference-manual.md
@@ -1984,8 +1984,8 @@ the following methods.
   *used as the `script_name` parameter.
 
 - `backend()` *(since 0.37.0)*: returns a string representing the
-  current backend: `ninja`, `vs2010`, `vs2015`, `vs2017`, `vs2019`,
-  or `xcode`.
+  current backend: `ninja`, `vs2010`, `vs2012`, `vs2013`, `vs2015`,
+  `vs2017`, `vs2019`, or `xcode`.
 
 - `build_root()`: returns a string with the absolute path to the build
   root directory. *(deprecated since 0.56.0)*: this function will return the
diff --git a/docs/markdown/snippets/newvsbackends.md b/docs/markdown/snippets/newvsbackends.md
new file mode 100644
index 0000000..cb7437a
--- /dev/null
+++ b/docs/markdown/snippets/newvsbackends.md
@@ -0,0 +1,15 @@
+## New `vs2012` and `vs2013` backend options
+
+Adds the ability to generate Visual Studio 2012 and 2013 projects.  This is an
+extension to the existing Visual Studio 2010 projects so that it is no longer
+required to manually upgrade the generated Visual Studio 2010 projects.
+
+Generating Visual Studio 2010 projects has also been fixed since its developer
+command prompt does not provide a `%VisualStudioVersion%` envvar.
+
+## Developer environment
+
+Expand the support for the `link_whole:` project option for pre-Visual Studio 2015
+Update 2, where previously Visual Studio 2015 Update 2 or later was required for
+this, for the Ninja backend as well as the vs2010 (as well as the newly-added
+vs2012 and vs2013 backends).