Handling the result for enum_from_name() is difficult, as checking for
negative return values requires a cast if the enum type is unsigned. The new
signature clearly differentiates lookup result from lookup value.
Further, this actually allows to convert real -1 enum values, which could not
be distinguished from "not-found" and the -1 return value.
This also fixes several clang warnings where enums are unsigned.
INCLUDES are now deprecated and throw warnings when using automake 1.13.
We now also differentiate AM_CPPFLAGS and AM_CFLAGS, where includes and
defines are passed to AM_CPPFLAGS only.
Tobias Brunner