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 <emil.velikov@collabora.com>
diff --git a/configure.ac b/configure.ac
index 4761c59..3f21cd5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -823,8 +823,11 @@
 fi
 dnl AX_PTHREADS leaves PTHREAD_LIBS empty for gcc and sets PTHREAD_CFLAGS
 dnl to -pthread, which causes problems if we need -lpthread to appear in
-dnl pkgconfig files.
-test -z "$PTHREAD_LIBS" && PTHREAD_LIBS="-lpthread"
+dnl pkgconfig files.  Since Android doesn't have a pthread lib, this check
+dnl is not valid for that platform.
+if test "x$android" = xno; then
+    test -z "$PTHREAD_LIBS" && PTHREAD_LIBS="-lpthread"
+fi
 
 dnl pthread-stubs is mandatory on targets where it exists
 case "$host_os" in