fix m4 dealing with split lines

Wei-keng Liao fixed this for pnetcdf. See pnetcdf revision 1904.
synclines and IF macro can interract poorly

See parallel-netcdf revision 1912:
http://trac.mcs.anl.gov/projects/parallel-netcdf/changeset/1912

the IF macro and scanlines can interact in a way that makes gcc unhappy
sync lines for generated C files

m4 can emit 'sync lines' which tell the toolchain "hey, this code
actually came from this other file over here".  Should help prevent
anyone accidentally editing a generated file.
Merge branch 'scanlines' of https://github.com/roblatham00/netcdf-c into roblatham00-scanlines
Merge branch 'roblatham00-scanlines'