Specify debian file format and compression type

Recent dpkg-deb versions create debian packages which are not compatible
with the Cydia tools by default. Specifying a debian package format and
the compression type should ensure compatibility.
Use plistutil instead of plutil

In recent libplist versions, the utility for compiling plist files is no
longer called plutil, but plistutil
Correct preprocessor condition to check for SDL2

The SDL_HINT_IDLE_TIMER_DISABLED hint was not included, because the
preprocessor condition "SDL_MAJOR_VERSION > 2" is not fulfilled with
SDL2.
Explicitly enable the OpenGL ES context profile for SDL2, if compiling for OpenGL ES

This makes it possible to use OpenGL ES on platforms where OpenGL and
OpenGL ES are available.
Do not use the FULL_VERSION variable for xcsoar-testing Cydia package version

The git commit version suffix can confuse dpkg, because it cannot
correctly determine if a package version is higher / lower than another.
Enable libcurl for iOS and Mac OS X

libcurl is now included in the darwin toolchain library set as a static
library for iOS. On Mac OS X, use the libcurl which is included in the
OS and the SDK.
Makefile: define macro USE_GLSL on GLES 2.0

Prepare for GLSL support on "desktop" OpenGL.