parse-datetime: fix a build failure with an older bison version

[Bruno Haible]

With an older bison version (2.3), a testdir for the 'parse-datetime'
module exhibits a build failure once the .y file has been modified:

$ make
$ cd gllib
$ touch *.y
$ make
: -d   ./parse-datetime.y \
&& sed -e 's|".*/parse-datetime.y"|"parse-datetime.y"|' < parse-datetime.tab.c 
> parse-datetime.c-t \
&& rm -f parse-datetime.tab.c parse-datetime.tab.h \
&& mv parse-datetime.c-t ./parse-datetime.c
/bin/bash: Zeile 1: parse-datetime.tab.c: Datei oder Verzeichnis nicht gefunden
make: *** [Makefile:1493: parse-datetime.c] Fehler 1

This patch fixes it. (Recall that some people unpack tarballs in ways
that assign arbitrary modification dates; they should not need a recent
version of bison in order to perform the build.)


2020-05-01  Bruno Haible  <address@hidden>

        parse-datetime: Fix a build failure with an older bison version.
        * modules/parse-datetime (Makefile.am): Don't do the post-processing of
        parse-datetime.tab.c if a suitable version of bison was not found.

diff --git a/modules/parse-datetime b/modules/parse-datetime
index 7571e06..b4375fd 100644
--- a/modules/parse-datetime
+++ b/modules/parse-datetime
@@ -44,9 +44,11 @@ Makefile.am:
 # Therefore we override this rule.
 parse-datetime.c: parse-datetime.y
        $(AM_V_YACC)$(PARSE_DATETIME_BISON) -d $(YFLAGS) $(AM_YFLAGS) 
$(srcdir)/parse-datetime.y \
-       && sed -e 's|".*/parse-datetime.y"|"parse-datetime.y"|' < 
parse-datetime.tab.c > parse-datetime.c-t \
-       && rm -f parse-datetime.tab.c parse-datetime.tab.h \
-       && mv parse-datetime.c-t $(srcdir)/parse-datetime.c
+       && test ':' = '$(PARSE_DATETIME_BISON)' || { \
+         sed -e 's|".*/parse-datetime.y"|"parse-datetime.y"|' < 
parse-datetime.tab.c > parse-datetime.c-t \
+         && rm -f parse-datetime.tab.c parse-datetime.tab.h \
+         && mv parse-datetime.c-t $(srcdir)/parse-datetime.c; \
+       }
 lib_SOURCES += parse-datetime.y
 BUILT_SOURCES += parse-datetime.c
 MOSTLYCLEANFILES += parse-datetime.tab.c parse-datetime.tab.h 
parse-datetime.c-t