Chain build commands using && instead of ;

This makes sure the build aborts as soon as possible if there is an
error, instead of having the error message drown in all the
following output.
Don't quote the android building commands in sh -c ''

The full command line can be written as such without any extra
wrapping.
Don't use LOCAL_PATH in LOCAL_SRC_FILES

These files are automatically relative to the directory of the
current makefile.
Run ndk-build from the android project top level directory

One doesn't need to run the ndk-build command from within the
jni subdirectory, calling it from the project top level directory
works just as well.

Also remove an unnecessary step of cd ../../../.. at the end -
since all make commands are run in a subshell, we don't need to
restore the current working directory afterwards.
Pass the right -mfpu flags for the armeabi-v7a target without NEON
Fix building the gtest library when targeting android

The main purpose is to make sure that the build doesn't fail
if the build directory contains the gtest subdirectory, although
the unittests also could be run on a device.
Set the CC and AR tools when building for android as well
Don't add C++ specific compiler flags to CFLAGS, add them to CXXFLAGS
Move LDFLAGS specific to creating a shared library to SHLDFLAGS
Merge pull request #330 from mstorsjo/android-build-fixes

Android build fixes/cleanups