Log Message:
libcerf: updated to 1.11

libcerf-1.11:
- Cover voigt by test_voigt
- Implement new function voigt_hwhm
- Restore libcerf.pc
- Add INSTALL instructions, and other minor adjustments for use of libcerf in \ 
C++ projects
- Support 'ctest', which runs the numeric accuracy tests from test1.c.
- Rename type cmplx into _cerf_cmplx to avoid name clash with Gnuplot pre 5.3