contrib: remove pywinmake submodule

The submodule may need frequent bumps and should be managed
separately as a build tool by developers. This commit also adjusts
the CMakeLists.txt to restrict the generated build configurations
on Windows to only "Release" because as this is the only config
that is currently used and functional (PDBs are generated and other
debug mechanisms can be activated as needed).

Build instructions are added to the README.

Gitlab: #1027
Change-Id: I367a6ded1a53056bc862c37d8e64417415cc339f

.gitmodules

@@ -1,4 +0,0 @@
-[submodule "extras/scripts/pywinmake"]
-	path = extras/scripts/pywinmake
-	url = https://review.jami.net/pywinmake
-	branch = master

CMakeLists.txt

@@ -351,6 +351,8 @@ if(MSVC)
    # Target
    ################################################################################
    include("CMake/Utils.cmake")
+   # Only generate Release configuration
+   set(CMAKE_CONFIGURATION_TYPES "Release" CACHE STRING "Limited configurations" FORCE)
    use_props(${PROJECT_NAME} "${CMAKE_CONFIGURATION_TYPES}" "${DEFAULT_CXX_PROPS}")
 
    ################################################################################
@@ -361,7 +363,6 @@ if(MSVC)
 
    # Supress generation of ZERO_CHECK
    set(CMAKE_SUPPRESS_REGENERATION true)
-   set(ROOT_NAMESPACE ringdaemon)
 
    ################################################################################
    # Output directory

README.md

@@ -168,6 +168,26 @@ Or, use your favorite package manager to install the necessary tools
 (macports or brew):
 `automake pkg-config libtool gettext yasm`
 
+How to compile on Windows
+----
+
+First, obtain and install `pywinmake` which is used to build the dependencies.
+
+```bash
+git clone "https://review.jami.net/pywinmake"
+cd pywinmake
+python -m pip install .
+```
+
+The rest of the build process uses CMake.
+
+```bash
+mkdir build
+cd build
+cmake ..
+cmake --build . --config Release
+```
+
 # Compile the dependencies
 ```bash
 cd contrib