BUILD,gflags.bzl: Fix android linking

Android is a special case where there is no libpthread.so to link against, although the symbols are present in the standard libaries.
  Linking against libpthread.so therefore causes an error.
  This fix uses the config method based on android toolchain that protobuf uses, which also omits -lpthread for linkopts.
    d16bf914bc/BUILD (L103)

BUILD

@@ -14,6 +14,11 @@ config_setting(
     values = {"cpu": "x64_windows"},
 )
 
+config_setting(
+    name = "android",
+    values = {"crosstool_top": "//external:android/crosstool"},
+)
+
 load(":bazel/gflags.bzl", "gflags_library", "gflags_sources")
 
 (hdrs, srcs) = gflags_sources(namespace = [

bazel/gflags.bzl

@@ -90,6 +90,7 @@ def gflags_library(hdrs = [], srcs = [], threads = 1):
     linkopts = []
     if threads:
         linkopts += select({
+            "//:android": [],
             "//:x64_windows": [],
             "//conditions:default": ["-lpthread"],
         })