test_libvpx: disable pthreads in gtest

currently threading is internal to libvpx so thread safety is unneeded
in libgtest -- visual studio builds already operate in this way as they
do not have pthread.h available by default.

this removes an unconditional link to libpthread using $(extralibs)
should libvpx require it.

Change-Id: Ieae1d693406653a54b54fba818c598836797d33b

configure

@@ -683,7 +683,7 @@ process_toolchain() {
             # x86 targets.
         ;;
         *)
-            enabled pthread_h && check_cxx "$@" <<EOF && soft_enable unit_tests
+            check_cxx "$@" <<EOF && soft_enable unit_tests
 int z;
 EOF
         ;;

libs.mk

@@ -442,6 +442,7 @@ else
 include $(SRC_PATH_BARE)/third_party/googletest/gtest.mk
 GTEST_SRCS := $(addprefix third_party/googletest/src/,$(call enabled,GTEST_SRCS))
 GTEST_OBJS=$(call objs,$(GTEST_SRCS))
+$(GTEST_OBJS) $(GTEST_OBJS:.o=.d): CXXFLAGS += -DGTEST_HAS_PTHREAD=0
 $(GTEST_OBJS) $(GTEST_OBJS:.o=.d): CXXFLAGS += -I$(SRC_PATH_BARE)/third_party/googletest/src
 $(GTEST_OBJS) $(GTEST_OBJS:.o=.d): CXXFLAGS += -I$(SRC_PATH_BARE)/third_party/googletest/src/include
 OBJS-$(BUILD_LIBVPX) += $(GTEST_OBJS)
@@ -466,7 +467,7 @@ $(foreach bin,$(LIBVPX_TEST_BINS),\
         lib$(CODEC_LIB)$(CODEC_LIB_SUF) libgtest.a ))\
     $(if $(BUILD_LIBVPX),$(eval $(call linkerxx_template,$(bin),\
         $(LIBVPX_TEST_OBJS) \
-        -L. -lvpx -lgtest -lpthread -lm)\
+        -L. -lvpx -lgtest $(extralibs) -lm)\
         )))\
     $(if $(LIPO_LIBS),$(eval $(call lipo_bin_template,$(bin))))\