Fail fast with verbose error when trying to native compile on non-ARM

Signed-off-by: Robin Vobruba <hoijui.quaero@gmail.com>

CMakeLists.txt

@@ -184,6 +184,13 @@ if(ANDROID_PLATFORM AND ANDROID_DEMO)
     add_subdirectory(android/NE10Demo/jni)
 endif()
 
+# Make sure we are compiling for an ARM system.
+# This is a verbose fail-fast in case we are trying to compile for non-ARM;
+# otherwise it would fail at `make` with obscure errors.
+if(GNULINUX_PLATFORM AND (NOT CMAKE_SYSTEM_PROCESSOR MATCHES "^arm"))
+    message(FATAL_ERROR "You are trying to compile for non-ARM (CMAKE_SYSTEM_PROCESSOR='${CMAKE_SYSTEM_PROCESSOR}')! see doc/building.md for cross compilation instructions.")
+endif()
+
 if(IOS_PLATFORM AND IOS_DEMO)
     add_subdirectory(ios)
 endif()

GNUlinux_config.cmake

@@ -36,6 +36,8 @@
 #     armv7 or aarch64 (Not done yet). Defaut is armv7.
 
 set(GNULINUX_PLATFORM ON)
+set(CMAKE_SYSTEM_NAME "Linux")
+set(CMAKE_SYSTEM_PROCESSOR "arm")
 
 if(NOT DEFINED ENV{NE10_LINUX_TARGET_ARCH})
    set(NE10_LINUX_TARGET_ARCH "armv7")