CMakeLists: add default "x64-windows" arch for Visual Studio

Commit 2d94b77 (cmake: allow building for Windows/ARM64, 2020-12-04)
failed to notice that /compat/vcbuild/vcpkg_install.bat will default
to the "x64-windows" architecture for the vcpkg installation if not set,
but CMake is not told of this default. Commit 635b6d9 (vcbuild: install
ARM64 dependencies when building ARM64 binaries, 2020-01-31) later updated
vcpkg_install.bat to accept an arch (%1) parameter, but retains the default.

This default is neccessary for the use case where the project directory is
opened firectly in Visual Studio, which will find and build a CMakeLists.txt
file without any parameters, thus expecting use of the default setting.

Visual studio will generate internal .sln solution and .vcproj project files
needed for some extension tools. Inform users of the additional .sln/.vcproj
generation.

How to test:
 rm -rf '.vs' # remove old visual studio settings
 rm -rf 'compat/vcbuild/vcpkg' # remove any vcpkg downloads
 with a fresh Visual Studio Community Edition, File>>Open>>(git folder)
  to load the project (which will take some time!).
 check for successful compilation.
The implicit .sln (etc.) are in the hidden .vs directory created by
Visual Studio.

Signed-off-by: Philip Oakley <[email protected]>

Author: PhilipOakley

contrib/buildsystems/CMakeLists.txt

@@ -27,7 +27,7 @@ Instructions to run CMake manually:
 
 This will build the git binaries in contrib/buildsystems/out
 directory (our top-level .gitignore file knows to ignore contents of
-this directory).
+this directory). The project .sln and .vcproj files are also generated.
 
 Possible build configurations(-DCMAKE_BUILD_TYPE) with corresponding
 compiler flags
@@ -71,6 +71,10 @@ if(USE_VCPKG)
 		message("Initializing vcpkg and building the Git's dependencies (this will take a while...)")
 		execute_process(COMMAND ${CMAKE_SOURCE_DIR}/compat/vcbuild/vcpkg_install.bat ${VCPKG_ARCH})
 	endif()
+	if(NOT EXISTS ${VCPKG_ARCH})
+		message("VCPKG_ARCH: unset, using 'x64-windows'")
+		set(VCPKG_ARCH "x64-windows") # default from vcpkg_install.bat
+	endif()
 	list(APPEND CMAKE_PREFIX_PATH "${VCPKG_DIR}/installed/${VCPKG_ARCH}")
 
 	# In the vcpkg edition, we need this to be able to link to libcurl