cmake: Don't link directly against a libpthread on Android (thanks, Anthony!).
Android has pthreads, but it's just part of their C runtime instead of a
separate library like the usual Linux platforms.
Fixes Bugzilla #3675.
1.1 --- a/cmake/sdlchecks.cmake Thu Jun 08 22:07:55 2017 +0300
1.2 +++ b/cmake/sdlchecks.cmake Wed Aug 09 19:50:18 2017 -0400
1.3 @@ -856,7 +856,10 @@
1.4 # PTHREAD_LIBS
1.5 macro(CheckPTHREAD)
1.6 if(PTHREADS)
1.7 - if(LINUX)
1.8 + if(ANDROID)
1.9 + # the android libc provides built-in support for pthreads, so no
1.10 + # additional linking or compile flags are necessary
1.11 + elseif(LINUX)
1.12 set(PTHREAD_CFLAGS "-D_REENTRANT")
1.13 set(PTHREAD_LDFLAGS "-pthread")
1.14 elseif(ANDROID)