Build system for Android target

[bennylp]

Adjust existing build system to support Android or create a new one that is based on Android.mk.

[ming]

In 4192:

Re #1516: Build system for Android
How to use:

• export ANDROID_NDK=/path_to_android_ndk_dir
• Run ./configure-android

[ming]

In 4195:

Re #1516: add configure-android file

[ming]

In 4202:

Re #1516: Add automatic Android API level detection and option to use ndk-build compilation flags

[ming]

In 4211:

Re #1516: Add sample config for Android build
Please refer to ​Android Getting Started guide

[ming]

In 4221:

Re #1516: Add help info for configure-android, add support for selecting target architecture

[ming]

In 4222:

Re #1516: Fixed compatibility problem with configure script on Ubuntu

[ming]

In 4594:

Re #1516: Add support to build for other targets. Example:
TARGET_ABI=armeabi-v7a ./configure-android --use-ndk-cflags

[nanang]

In 4599:

Re #1516: Fixed configure-android in probing toolchain dir (reported that on ndk-r8, there is also 'gdbserver' dir under 'toolchains/arm/prebuilt').

[ming]

In 4604:

Re #1516: Add feature to ignore certain compilation flags from the --use-ndk-cflags option.

[ming]

In 4651:

Re #1516: Set AR and RANLIB (thanks to Brian Walker for the patch)

[nanang]

In 4689:

Re #1516: Avoid duplicated LDFLAGS in pjlib-test, which may raise linking errors (e.g: on Android).

[bennylp]

In 4732:

Re #1516: fixing multiple inclusions of crtbegin.o and -lgcc in the makefiles. The former causes link errors due to multiple symbols