Note: The Autoconf project has a Makefile.maint that contains this target:

# Verify that all source files using _() are listed in po/POTFILES.in.
# FIXME: don't hard-code file names below; use a more general mechanism.
po-check:
        if test -f po/POTFILES.in; then                                        \
          grep -E -v '^(#|$$)' po/POTFILES.in                                \
            | grep -v '^src/false\.c$$' | sort > $@-1;                        \
          files=;                                                        \
          for file in $$($(CVS_LIST_EXCEPT)) lib/*.[ch]; do                \
            case $$file in                                                \
            djgpp/* | man/*) continue;;                                        \
            esac;                                                        \
            case $$file in                                                \
            *.[ch])                                                        \
              base=`expr " $$file" : ' \(.*\)\..'`;                        \
              { test -f $$base.l || test -f $$base.y; } && continue;;        \
            esac;                                                        \
            files="$$files $$file";                                        \
          done;                                                                \
          grep -E -l '\bN?_\([^)"]*("|$$)' $$files | sort -u > $@-2;        \
          diff -u $@-1 $@-2 || exit 1;                                        \
          rm -f $@-1 $@-2;                                                \
        fi

And the gnulib/top/maint.mk contains the target sc_po_check, that does similar things:

# Verify that all source files using _() (more specifically, files that
# match $(_gl_translatable_string_re)) are listed in po/POTFILES.in.
po_file ?= $(srcdir)/po/POTFILES.in
generated_files ?= $(srcdir)/lib/*.[ch]
_gl_translatable_string_re ?= \b(N?_|gettext *)\([^)"]*("|$$)
sc_po_check:
        @if test -f $(po_file); then                                        \
          grep -E -v '^(#|$$)' $(po_file)                                \
            | grep -v '^src/false\.c$$' | sort > $@-1;                        \
          files=$$(perl $(perl_translatable_files_list_)                \
            $$($(VC_LIST_EXCEPT)) $(generated_files));                        \
          grep -E -l '$(_gl_translatable_string_re)' $$files                \
            | $(SED) 's|^$(_dot_escaped_srcdir)/||' | sort -u > $@-2;        \
          diff -u -L $(po_file) -L $(po_file) $@-1 $@-2                        \
            || { printf '$(ME): '$(fix_po_file_diag) 1>&2; exit 1; };        \
          rm -f $@-1 $@-2;                                                \
        fi

The last big missing feature from intltool is detecting missing and non-existing files in POTFILES.in/.skip (intltool-update -m). It would be great if a similar feature could be implemented in gettext.