android: integrate ndk-build step into main gradle pipeline

this greatly simplifies the assembly of the android application. moreover, ndk
will be downloaded automatically via sdk-manager.

Makefile.am

 
 ##### android targets:
 
-jni:
-	cd android && ndk-build
-
-apk-release: jni espeakdata
+apk-release: espeakdata
 	cd android && $(GRADLE) assembleRelease
 
-apk-debug: jni espeakdata
+apk-debug: espeakdata
 	cd android && $(GRADLE) assembleDebug
 
-apk-check: jni espeakdata
+apk-check: espeakdata
 	cd android && $(GRADLE) connectedCheck
 
 android/res/raw/espeakdata.zip: espeak-ng-data/phontab dictionaries

android/build.gradle

 }
 
 android {
-    buildToolsVersion '28.0.3'
     compileSdkVersion 26
 
     defaultConfig {
         versionName "1.52-dev"
     }
 
+    externalNativeBuild {
+        ndkBuild {
+            path "jni/Android.mk"
+        }
+    }
+
     sourceSets {
         main {
             manifest.srcFile 'AndroidManifest.xml'
             java.srcDirs = ['src']
             res.srcDirs = ['res']
-            jniLibs.srcDir 'libs'
         }
         androidTest.setRoot('eSpeakTests')
         androidTest {

android/jni/Application.mk

 APP_OPTIM             := release
-APP_PLATFORM          := android-26
-APP_TOOLCHAIN_VERSION := 4.6.3
-APP_ABI               := armeabi-v7a x86

configure.ac

 AC_PROG_LIBTOOL
 AC_PROG_LN_S
 
-AC_CHECK_PROG(NDKBUILD_CHECK,[ndk-build],yes)
-
 dnl ================================================================
 dnl gradle checks.
 dnl ================================================================
         PCAudioLib:                    ${have_pcaudiolib}
 
         gradle (Android):              ${GRADLE}
-        ndk-build (Android):           ${NDKBUILD_CHECK}
 
         Klatt:                         ${have_klatt}
         speechPlayer:                  ${have_speechplayer}