avoid -lpthread link option when building with emscripten without threads

- having this flag causes emscripten to generate binary with pthread
  support, which is not compatible with browsers that do not support
  pthreads or with web pages that do not enable pthread support

BUILD.bazel

@@ -56,6 +56,16 @@ config_setting(
     constraint_values = ["@platforms//os:openbsd"],
 )
 
+config_setting(
+    name = "emscripten_without_threads",
+    constraint_values = [
+        "@platforms//os:emscripten",
+    ],
+    values = {
+        "features": "-use_pthreads",
+    },
+)
+
 # NOTE: Fuchsia is not an officially supported platform.
 config_setting(
     name = "fuchsia",
@@ -111,10 +121,12 @@ cc_library(
     copts = select({
         ":qnx": [],
         ":windows": [],
+        ":emscripten_without_threads": [],
         "//conditions:default": ["-pthread"],
     }),
     defines = select({
         ":has_absl": ["GTEST_HAS_ABSL=1"],
+        ":emscripten_without_threads": ["GTEST_HAS_PTHREAD=0"],
         "//conditions:default": [],
     }),
     features = select({
@@ -138,6 +150,7 @@ cc_library(
             "-lm",
             "-pthread",
         ],
+        ":emscripten_without_threads": [],
         "//conditions:default": ["-pthread"],
     }),
     deps = select({