configure.ac: Don't look for pthreads in Android platform
configure.ac: Don't look for pthreads in Android platform

In Android, the pthreads libs are in bionic.  When building
Mesa for Android with the autotools workflow, we shouldn't
set -lpthread or -pthread.

[Emil Velikov]
Other platforms could use a similar fix, although that is left as
separate exercise.

Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler