CXXFLAGS should not include -std=gnu99
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m4/devil-definitions.m4 adds "-std=gnu99 -fgnu99-inline" calls ADD_CFLAGS(-std=gnu99 -fgnu89-inline).
lib/Makefile.am contains the line "libIL_la_CXXFLAGS = $(AM_CFLAGS) @IL_CFLAGS@".
This means that C++ code will be compiled with -std=gnu99. This makes no sense because that's a C standard, not a C++ standard and clang++ rightly rejects it.
The first line of this report should have read:
m4/devil-definitions.m4 calls ADD_CFLAGS(-std=gnu99 -fgnu89-inline).