configure: Move ocaml-gettext writing boilerplate to separate m4 module.

This is just code motion.

NOTE that after this commit you have to do:

  aclocal -I m4
  autoconf

as 'make' may not do this automatically.
builder: Suppress warning about unused yyunput.

  CC       virt_index_validate-index-validate.o
index-scan.c:1270:17: warning: 'yyunput' defined but not used [-Wunused-function]
     static void yyunput (int c, register char * yy_bp )
                 ^
  CCLD     virt-index-validate

By using %option nounput we can prevent this from being generated in
the first place.
Use bindtextdomain in some programs where it was missing.

Found by make syntax-check.
src/drives.c: Remove useless if-before-free.

Found by make syntax-check.
Remove some unused header files.

Found by make syntax-check.
test: Use = instead of == for testing string equality in bash script.

Found by make syntax-check.