Disabled getrandom provider on Android prior to API version 28.

Android supports getrandom and getentropy functions only since API version 28.

As part of the fix converted Linux/Android platform detection from Boost.Predef
to direct checks of platform-specific macros. This is to work around
Boost.Predef problem described in [1] - depending on header inclusion order
Boost.Predef may indicate Linux or Android. When that problem is fixed we
may change back to Boost.Predef.

Fixes boostorg#76.

[1]: boostorg/predef#81 (comment)

include/boost/uuid/detail/random_provider_detect_platform.hpp

@@ -14,10 +14,15 @@
 #include <boost/predef/library/c/cloudabi.h>
 #include <boost/predef/library/c/gnu.h>
 #include <boost/predef/os/bsd/open.h>
-#include <boost/predef/os/linux.h>
 #include <boost/predef/os/windows.h>
-#if BOOST_OS_LINUX
+
+// Note: Don't use Boost.Predef to detect Linux and Android as it may give different results depending on header inclusion order.
+// https://github.com/boostorg/predef/issues/81#issuecomment-413329061
+#if (defined(__linux__) || defined(__linux) || defined(linux)) && (!defined(__ANDROID__) || __ANDROID_API__ >= 28)
 #include <sys/syscall.h>
+#if defined(SYS_getrandom)
+#define BOOST_UUID_RANDOM_PROVIDER_HAS_GETRANDOM
+#endif // defined(SYS_getrandom)
 #endif
 
 //
@@ -45,7 +50,7 @@
 #  error Unable to find a suitable windows entropy provider
 # endif
 
-#elif BOOST_OS_LINUX && defined(SYS_getrandom) && !defined(BOOST_UUID_RANDOM_PROVIDER_FORCE_POSIX) && !defined(BOOST_UUID_RANDOM_PROVIDER_DISABLE_GETRANDOM)
+#elif defined(BOOST_UUID_RANDOM_PROVIDER_HAS_GETRANDOM) && !defined(BOOST_UUID_RANDOM_PROVIDER_FORCE_POSIX) && !defined(BOOST_UUID_RANDOM_PROVIDER_DISABLE_GETRANDOM)
 # define BOOST_UUID_RANDOM_PROVIDER_GETRANDOM
 # define BOOST_UUID_RANDOM_PROVIDER_NAME getrandom