Add missing epoxy cmake file

indra/cmake/Epoxy.cmake

+# -*- cmake -*-
+include(Prebuilt)
+
+if (USESYSTEMLIBS)
+  include(FindPkgConfig)
+
+  pkg_check_modules(EPOXY REQUIRED epoxy)
+else (USESYSTEMLIBS)
+  use_prebuilt_binary(libepoxy)
+  set(EPOXY_INCLUDE_DIRS ${LIBS_PREBUILT_DIR}/include/)
+  set(EPOXY_LIBRARIES epoxy)
+endif (USESYSTEMLIBS)
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