For Android there is no need to expicitly link pthread lib.

Change-Id: Ifbd1c6fb91a9e8c6e5d50aa2b2ff7684fbb248a9
See: https://groups.google.com/forum/#!topic/android-ndk/Dq05en_xoN8
Reviewed-on: https://boringssl-review.googlesource.com/22084
Reviewed-by: Matt Braithwaite <mab@google.com>
Commit-Queue: Matt Braithwaite <mab@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

util/BUILD.toplevel

@@ -46,6 +46,11 @@ config_setting(
     values = {"cpu": "x64_windows"},
 )
 
+config_setting(
+    name = "android",
+    values = {"crosstool_top": "//external:android/crosstool"}
+)
+
 posix_copts = [
     # Assembler option --noexecstack adds .note.GNU-stack to each object to
     # ensure that binaries can be built with non-executable stack.
@@ -120,6 +125,9 @@ cc_library(
     includes = ["src/include"],
     linkopts = select({
         ":mac_x86_64": [],
+        # Android supports pthreads, but does not provide a libpthread
+        # to link against.
+        ":android": [],
         "//conditions:default": ["-lpthread"],
     }),
     visibility = ["//visibility:public"],