Visual Studio builds: "Add" Visual Studio 2019 projects

Copy and update the relevant fields from the Visual Studio 2010 projects
so that we can have project files that work out-of-the-box for Visual
Studio 2019, as we did for Visual Studio 2012 through 2017.

Also update the NMake Makefiles for building the introspection files
so that we properly detect that we are building with Visual Studio 2019.

build/win32/Makefile.am

@@ -31,7 +31,8 @@ SUBDIRS =	\
 	vs11	\
 	vs12	\
 	vs14	\
-	vs15
+	vs15	\
+	vs16
 
 EXTRA_DIST +=	\
 	detectenv-msvc.mak		\

build/win32/detectenv-msvc.mak

@@ -42,8 +42,10 @@ VSVER = 11
 VSVER = 12
 !elseif $(VCVERSION) > 1899 && $(VCVERSION) < 1910
 VSVER = 14
-!elseif $(VCVERSION) > 1909 && $(VCVERSION) < 2000
+!elseif $(VCVERSION) > 1909 && $(VCVERSION) < 1920
 VSVER = 15
+!elseif $(VCVERSION) > 1919 && $(VCVERSION) < 2000
+VSVER = 16
 !else
 VSVER = 0
 !endif
@@ -51,7 +53,7 @@ VSVER = 0
 !if "$(VSVER)" == "0"
 MSG = ^
 This NMake Makefile set supports Visual Studio^
-9 (2008) through 14 (2015).  Your Visual Studio^
+9 (2008) through 16 (2019).  Your Visual Studio^
 version is not supported.
 !error $(MSG)
 !endif

build/win32/vs16/Makefile.am

@@ -0,0 +1,34 @@
+include $(top_srcdir)/Makefile.decl
+
+EXTRA_DIST +=	\
+	README.txt	\
+	gtk+.sln	\
+	gtk-build-defines.props	\
+	gtk-gen-srcs.props	\
+	gtk-install.props	\
+	gtk-version-paths.props	\
+	gdk-win32.vcxproj	\
+	gdk-win32.vcxproj.filters	\
+	gdk.vcxproj	\
+	gdk.vcxproj.filters	\
+	gtk.vcxproj	\
+	gtk.vcxproj.filters	\
+	gtk-demo.vcxproj	\
+	gtk-demo.vcxproj.filters	\
+	gailutil.vcxproj	\
+	gailutil.vcxproj.filters	\
+	libwimp.vcxproj	\
+	libwimp.vcxproj.filters	\
+	gtk-update-icon-cache.vcxproj	\
+	gtk-update-icon-cache.vcxproj.filters	\
+	gtk-install.vcxproj	\
+	gtk-install.vcxproj.filters
+
+DISTCLEANFILES = $(EXTRA_DIST)
+
+MSVC_VER = 16
+MSVC_VER_LONG = 16
+MSVC_TOOLSET = 142
+MSVC_FORMAT_VER = 12
+
+include $(top_srcdir)/build/Makefile-newvs.am