Add -d:androidNDK to fix echo on Android NDK builds. (#12203)

* Add -d:echoToAndroidLog to fix echo.

* Change to androidNDK and add docs.

* Some word changes to docs.

doc/nimc.rst

@@ -281,6 +281,20 @@ The MinGW-w64 toolchain can be installed as follows::
   CentOS: yum install mingw32-gcc | mingw64-gcc - requires EPEL
   OSX: brew install mingw-w64
 
+Cross compilation for Android
+=============================
+
+There are two ways to compile for Android: terminal programs (Termux) and with the NDK (Android Native Development Kit).
+
+First one is to treat Android as a simple linux and use `Termux <https://wiki.termux.com>`_ to connect and run the nim compiler directly on android as if it was linux. These programs are console only apps
+that can’t be distributed in the Play Store.
+
+Use regular ``nim c`` inside termux to make Android terminal apps.
+
+Android apps are written in Java, to use Nim inside an Android app you need a small Java stub that calls out to a native library written in Nim using the `NDK <https://developer.android.com/ndk>`_. You can also use `native-acitivty <https://developer.android.com/ndk/samples/sample_na>`_ to have the Java stub be auto generated for you.
+
+Use ``nim c -c --cpu:arm --os:android -d:androidNDK`` to generate the C source files you need to include in your Android Studio project. Add the generated C files to CMake build script. Then do the final compile with Android Studio which uses gradle to call CMake to compile the project.
+
 Cross compilation for Nintendo Switch
 =====================================
 

lib/system/io.nim

@@ -112,6 +112,13 @@ proc c_setvbuf(f: File, buf: pointer, mode: cint, size: csize): cint {.
 proc c_fprintf(f: File, frmt: cstring): cint {.
   importc: "fprintf", header: "<stdio.h>", varargs, discardable.}
 
+## When running nim in android app stdout goes no where, so echo gets ignored
+## To redreict echo to the android logcat use -d:androidNDK
+when defined(androidNDK):
+  const ANDROID_LOG_VERBOSE = 2.cint
+  proc android_log_print(prio: cint, tag: cstring, fmt: cstring): cint
+    {.importc: "__android_log_print", header: "<android/log.h>", varargs, discardable.}
+
 template sysFatal(exc, msg) =
   raise newException(exc, msg)
 
@@ -585,6 +592,12 @@ when declared(stdout):
     initSysLock echoLock
 
   proc echoBinSafe(args: openArray[string]) {.compilerproc.} =
+    when defined(androidNDK):
+      var s = ""
+      for arg in args:
+        s.add arg
+      android_log_print(ANDROID_LOG_VERBOSE, "nim", s)
+    else:
       # flockfile deadlocks some versions of Android 5.x.x
       when not defined(windows) and not defined(android) and not defined(nintendoswitch):
         proc flockfile(f: File) {.importc, nodecl.}