Fix Mac checkbuild.

Bug: 21669400
Change-Id: I54691661605a2e2c1871781964aed343840b8560
(cherry-pick from commit 33c2d441b8f816722bed984c6e12b82b888c3ca2)
diff --git a/tools/relocation_packer/Android.mk b/tools/relocation_packer/Android.mk
index 75dba71..4e2fd97 100644
--- a/tools/relocation_packer/Android.mk
+++ b/tools/relocation_packer/Android.mk
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+ifeq ($(HOST_OS),linux)
 common_cppflags := -Wall -Wextra -Wunused -Werror -Wold-style-cast
 
 LOCAL_PATH := $(call my-dir)
@@ -95,3 +96,5 @@
 $(eval $(call copy-test-library,elf_file_unittest_relocs_arm32_packed.so))
 $(eval $(call copy-test-library,elf_file_unittest_relocs_arm64.so))
 $(eval $(call copy-test-library,elf_file_unittest_relocs_arm64_packed.so))
+
+endif