Update build process

[gemmaro]

Hello,

This is a continuation from #1, migrating from deprecated po4a-updatepo and po4a-translate to integrated po4a command.

The *.po, *.texi, and *.info files are not updated for this time.

Update: Add PO files to prerequisites:

diff --git a/docs/docs-ja/Makefile b/docs/docs-ja/Makefile
index 5887c8cf..d167f73f 100644
--- a/docs/docs-ja/Makefile
+++ b/docs/docs-ja/Makefile
@@ -52,7 +52,7 @@ $(PO4A_CONFIG):
        TRANSLATE_RAW_DIR=$(TRANSLATE_RAW_DIR)          \
                ./generate-po4a-config > $@
 
-$(TARGET_TRANSLATE_RAW_TEXI) &: $(TARGET_SED_OUT) $(PO4A_CONFIG)
+$(TARGET_TRANSLATE_RAW_TEXI) &: $(TARGET_SED_OUT) $(TARGET_PO) $(PO4A_CONFIG)
        po4a $(PO4A_CONFIG)
 
 $(TARGET_TEXI): %.ja.texi : $(TRANSLATE_RAW_DIR)/%.ja.texi

Update2: Correctly set pot specifiers for split mode, update PO files, and generate translations.

Best,