Add custom update-pot target

While working on a separate localization bug, I noticed that
`po/exiv2.pot` hasn't been updated since 2017, and that with the removal
of the autotools build system there's no longer any way to update it.
Add a simple `update-pot` target to do so, and bring `po/POTFILES.in` up
to date with the current source tree.

I haven't updated `po/exiv2.pot` itself because that would be a large
diff, though I've checked that the output looks reasonable.  I suggest
that maintainers do this.

po/CMakeLists.txt

@@ -19,3 +19,20 @@ GETTEXT_CREATE_TRANSLATIONS(exiv2.pot ALL
     uk.po
     vi.po
 )
+
+find_program(XGETTEXT xgettext)
+
+add_custom_target(
+    update-pot
+    COMMAND
+    ${XGETTEXT} -o ${CMAKE_CURRENT_SOURCE_DIR}/exiv2.pot
+        -D ${PROJECT_SOURCE_DIR}
+        --keyword=_ --keyword=N_
+        --files-from=${CMAKE_CURRENT_SOURCE_DIR}/POTFILES.in
+        --from-code=UTF-8
+        --copyright-holder='Andreas Huggel'
+        --msgid-bugs-address=ahuggel@gmx.net
+        --package-name=${CMAKE_PROJECT_NAME}
+        --package-version=${CMAKE_PROJECT_VERSION}
+    COMMENT "Extract translatable messages to exiv2.pot"
+)

po/Makevars

@@ -1,41 +0,0 @@
-# Makefile variables for PO directory in any package using GNU gettext.
-
-# Usually the message domain is the same as the package name.
-DOMAIN = $(PACKAGE)
-
-# These two variables depend on the location of this directory.
-subdir = po
-top_builddir = ..
-
-# These options get passed to xgettext.
-XGETTEXT_OPTIONS = --keyword=_ --keyword=N_
-
-# This is the copyright holder that gets inserted into the header of the
-# $(DOMAIN).pot file.  Set this to the copyright holder of the surrounding
-# package.  (Note that the msgstr strings, extracted from the package's
-# sources, belong to the copyright holder of the package.)  Translators are
-# expected to transfer the copyright for their translations to this person
-# or entity, or to disclaim their copyright.  The empty string stands for
-# the public domain; in this case the translators are expected to disclaim
-# their copyright.
-COPYRIGHT_HOLDER = Andreas Huggel
-
-# This is the email address or URL to which the translators shall report
-# bugs in the untranslated strings:
-# - Strings which are not entire sentences, see the maintainer guidelines
-#   in the GNU gettext documentation, section 'Preparing Strings'.
-# - Strings which use unclear terms or require additional context to be
-#   understood.
-# - Strings which make invalid assumptions about notation of date, time or
-#   money.
-# - Pluralisation problems.
-# - Incorrect English spelling.
-# - Incorrect formatting.
-# It can be your email address, or a mailing list address where translators
-# can write to without being subscribed, or the URL of a web page through
-# which the translators can contact you.
-MSGID_BUGS_ADDRESS = ahuggel@gmx.net
-
-# This is the list of locale categories, beyond LC_MESSAGES, for which the
-# message catalogs shall be used.  It is usually empty.
-EXTRA_LOCALE_CATEGORIES =

po/POTFILES.in

@@ -1,23 +1,24 @@
 # List of source files with translatable strings, generated with
 # $ grep -l '[N ]_(' src/*.cpp src/*.hpp | sort
-# Note: Please run 'make update-po' after changing this list
+# Note: Please run 'make update-pot' after changing this list
 
 src/actions.cpp
-src/canonmn.cpp
+src/canonmn_int.cpp
-src/crwimage.cpp
+src/casiomn_int.cpp
+src/crwimage_int.cpp
 src/datasets.cpp
 src/error.cpp
 src/exiv2.cpp
-src/fujimn.cpp
+src/fujimn_int.cpp
-src/minoltamn.cpp
+src/minoltamn_int.cpp
-src/nikonmn.cpp
+src/nikonmn_int.cpp
-src/olympusmn.cpp
+src/olympusmn_int.cpp
-src/panasonicmn.cpp
+src/panasonicmn_int.cpp
-src/pentaxmn.cpp
+src/pentaxmn_int.cpp
 src/properties.cpp
-src/sigmamn.cpp
+src/samsungmn_int.cpp
-src/sonymn.cpp
+src/sigmamn_int.cpp
+src/sonymn_int.cpp
 src/tags.cpp
-src/tiffimage.cpp
+src/tags_int.cpp
-src/tiffvisitor.cpp
+src/tiffimage_int.cpp
-src/webpimage.cpp