Add script to check that src/Makefile.am is up to date about erfa

[rmathar]

One way is

• move src/Makefile.am to src/Makefile.am.in
• delete the multi-line assignment of liberfa_la_SOURCES in src/Makefile.am.in
• add lines in configure.ac before the AC_CONFIG_FILES() with something
  that copies all c source files, excluding the test code:
  (cd Makefile ;
  cp Makefile.am.in Makefile.am ;
  echo liberfa_la_SOURCES = $(ls -w 4096 -t *.c *.h | grep -v t_erfa ) >> Makefile.am
  )

[eteq]

@rmathar - that sounds like a reasonable idea... would you like to make a PR that makes these changes? (since it's your idea that way you'd get credit for the commits).