Apparently clang does not recognize the
#pragma GCC visibility push(default)
lines in osl.h and complains for each file which includes this
header. Compliling without -Wno-unknown-pragmas fixes this.
DEBUG_CPPFLAGS += -g -Wunused -Wundef -W
DEBUG_CPPFLAGS += -Wredundant-decls
-DEBUG_CPPFLAGS += -Wall -Wno-sign-compare
+DEBUG_CPPFLAGS += -Wall -Wno-sign-compare -Wno-unknown-pragmas
DEBUG_CPPFLAGS += -Wformat-security
DEBUG_CPPFLAGS += -Wmissing-format-attribute
# produces false positives