Initial CMake support for libretroshare

libretroshare (not all build options yet) and it's dependencies can now
  be built using CMake instead of qmake.
Even Qt itself deprecated qmake, which is not developed anymore, as
  build system and it was making many things much more difficult
  and requiring an enormous amount of black magic to support a wide
  range of platforms.
libretroshare can now easly be build as static or shared library with
  simple commands and a maintaniable build system:
```
cmake \
	-D RS_LIBRETROSHARE_STATIC=OFF -D RS_LIBRETROSHARE_SHARED=ON \
	-S $YOUR_RS_SOURCE_DIR/libretroshare/ -B .
make
```

CMakeLists.txt

@@ -0,0 +1,19 @@
+# RetroShare decentralized communication platform
+#
+# Copyright (C) 2021  Gioacchino Mazzurco <gio@eigenlab.org>
+# Copyright (C) 2021  Asociación Civil Altermundi <info@altermundi.net>
+#
+# SPDX-License-Identifier: CC0-1.0
+
+cmake_minimum_required (VERSION 2.8.12)
+project(libbitdht)
+
+file(
+	GLOB BITDHT_SOURCES
+	src/bitdht/*.c src/bitdht/*.cc src/udp/*.cc src/util/*.cc )
+
+add_library(${PROJECT_NAME} ${BITDHT_SOURCES})
+
+target_include_directories(
+	${PROJECT_NAME}
+	PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src )