Doc building with CMake

Just a simple doc, should be better than nothing.

BUILD.cmake

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+Build with CMake
+
+Building with cmake supports building out of tree to clean the source tree intact.
+
+Native build (Unix Makefiles):
+#mkdir build
+#cmake [options] /path-to-ren-c/make
+Options support:
+R3_WITH_TCC: user natives depend on this, When this is enabled, CMAKE_BUILD_TYPE has to be defined as well
+R3_EXTERNAL_FFI: build with libffi provided by the system
+R3_OS_ID: the OS_ID with which Rebol 3 is to build
+#make -j8 r3-core
+
+Cross-compile for Windows from Linux (mingw64)
+Just change the cmake step to include the toolchain file, e.g. for 32-bit windows executable:
+#cmake [options] /path-to-ren-c/make -DCMAKE_TOOLCHAIN_FILE=/path-to-ren-c/make/Toolchain-cross-mingw32-linux.cmake
+
+Cross-compile for Android from Linux
+1. Preparation: 
+1.1 install 'android-ndk' 
+1.2 install 'android-cmake' 
+1.3 checkout the latest ren-c 
+
+2. compile a TCC cross-compiler: 
+2.1 #mkdir build; cd build 
+2.2 #cmake -DTCC_BUILD_ARM_EABIHF=1 -DTCC_ARM_VERSION=7 /path-to-ren-c/external/tcc 
+2.3 #make ;it should produce an exe arm-linux-gnueabifh-tcc 
+
+3. Compile Rebol 3: 
+3.1 #mkdir build; cd build 
+3.2 #copy arm-linux-gnueabifh-tcc (from step 2.3) ./cross-tcc 
+3.3 #copy your-r3-make ./r3-make
+3.4 #export ANDROID_NDK=/opt/android-ndk 
+3.5 #export ANDROID_NATIVE_API_LEVEL=16 
+3.6 #cmake -DR3_WITH_TCC=1 -DCMAKE_BUILD_TYPE=RELEASE -DR3_OS_ID=0.13.2 -DCMAKE_TOOLCHAIN_FILE=/opt/android-cmake/android.toolchain.cmake /path-to-ren-c/make
+3.7 #make r3-core