Make shared object use extralibs

this way -lm doesn't get ignored if additional LDFLAGS get passed from
env

Change-Id: Ie630369ae6ed2780377c35aa2726e759d527bb50

build/make/Makefile

diff --git a/build/make/Makefile b/build/make/Makefile
index ba2578e..4f7df43 100755
--- a/build/make/Makefile
+++ b/build/make/Makefile
@@ -231,7 +231,7 @@
 	$(qexec)$$(LD) -shared $$(LDFLAGS) \
             -Wl,--no-undefined -Wl,-soname,$$(SONAME) \
             -Wl,--version-script,$$(SO_VERSION_SCRIPT) -o $$@ \
-            $$(filter %.o,$$?)
+            $$(filter %.o,$$?) $$(extralibs)
 endef
 
 define lipo_lib_template

libs.mk

diff --git a/libs.mk b/libs.mk
index c6b08d2..fd4543b 100644
--- a/libs.mk
+++ b/libs.mk
@@ -182,7 +182,7 @@
 LIBVPX_SO               := libvpx.so.$(VERSION_MAJOR).$(VERSION_MINOR).$(VERSION_PATCH)
 LIBS-$(BUILD_LIBVPX_SO) += $(BUILD_PFX)$(LIBVPX_SO)
 $(BUILD_PFX)$(LIBVPX_SO): $(LIBVPX_OBJS) libvpx.ver
-$(BUILD_PFX)$(LIBVPX_SO): LDFLAGS += -lm -pthread
+$(BUILD_PFX)$(LIBVPX_SO): extralibs += -lm -pthread
 $(BUILD_PFX)$(LIBVPX_SO): SONAME = libvpx.so.$(VERSION_MAJOR)
 $(BUILD_PFX)$(LIBVPX_SO): SO_VERSION_SCRIPT = libvpx.ver
 LIBVPX_SO_SYMLINKS      := $(addprefix $(LIBSUBDIR)/, \