This makes the minimum amount of changes to allow inclusion of dtrace.h
without all the solaris compatibility headers. Installing dtrace.h allows
compiling consumers of libdtrace (e.g. https://github.com/tmetsch/python-dtrace)
without requiring a copy of the source tree.
For python-dtrace I worked around this in 58019c9a12
but being able to build the library without installed sources would be
extremely useful.
Reviewed By: gnn
Differential Revision: https://reviews.freebsd.org/D27884
1f6612b444