cmds: implement CPU feature sanity check via builtin compiler functions

To prevent our whilelist from growing much more we know do an additional
sanity check via compiler builtin functions if we cannot detect all
mandatory CPU features via the cpu_features libray and if the CPU isn't
whitelisted. This should cover more or less all remaining cases unless
the compiler is unable to detect the features properly.

Update check_features.cpp - Added override for Core i7-720QM

Add option to disable sdl window hit test with envar value

Update check_features.cpp - Added override for Intel Xeon E5520

I also get the same SSE error on a virtual machine with an Intel Xeon Processor:
The CPU of your computer (Intel(R) Xeon(R) CPU E5520 @ 2.27GHz) does not support all
features Anbox requires.
It is missing support for the following features: SSE 4.1, SSE 4.2, SSSE 3

but checking /proc/cpuinfo reveals that it supports the features:
...
model name	: Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
...
flags	: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx rdtscp lm constant_tsc rep_good nopl xtopology nonstop_tsc cpuid pni ssse3 sse4_1 sse4_2 x2apic hypervisor lahf_lm pti

I hope adding the following code into the array in check_features.cpp could fix the problem:
// Intel Xeon Processor E5520
"E5520"

Update check_features.cpp - Added override for Pentium T4500

Add probot stale configuration

We want to get issues without activity closed after a while unless they
are marked with specific labels to keep things clean and maintainable.

Fix clang build errors

cmds: add override for Intel Core i5 M 460

This commit uses GCC friendly pragmas to demote a clang error on
C99-specific feature usage to a warning.

The C99 feature that is used here is compound literals. It gets used
trough a macro from the dbus C library.

This removes the constexpr const char * default_glesv1_lib and promote
-Wunused-const-variable back to error level on clang.

Remove a unused lambda capture on [this] in
anbox::cmds::CheckFeatures::CheckFeatures() and promote
-Wunused-lambda-capture back to error level on clang.

This commit removes the private unused field type_ from anbox::dbus::Bus
and promotes -Wunused-private-field back to error level on clang.

cmds: add override for Intel Celeron N2840

This commit downgrades the following clang compiler errors into warnings:
 * unused-private-field
 * unused-const-variable
 * unused-lambda-capture

Change -Wcast-align from error to warning for clang compiler by adding
the -Wno-error=cast-align compiler flag.

Fixes #881

In some spare situations it might be useful to change the network
configuration of the bridge and with that also the configuration of
the Android container itself. We don't validate the configuration
and just apply it no matter if it ends up with network connectivity
in the container.