[PATCH v2] Support building development version without git

[Henri M. <hen...@gm...>]

Currently building the development version requires git to be installed
to fetch the `last modified' date from the commit information.  This is
okay when building from a git clone, but fails when downloading a
snapshot that does not contain the .git directory.

To this end, I implemented a fallback that uses the last modification
time of the Makefile.am, which corresponds to the last commit date when
using a fresh checkout.  The old version had the advantage that
`timestamp.h' would only be rebuilt when changing the git branch.  If
git is not available, this logic fails of course.  Instead I made
`timestamp.h' a phony target, i.e. it will be rebuilt unconditionally
every time.  The disadvantage here is that this will also trigger a
rebuild of every file that depends on `timestamp.h' but as far as I can
see this is only `version.c'.
---
This implements the suggestion by Miquel Garriga to use the mtime of
Makefile.am which is set to the last commit time on a fresh checkout.

 src/Makefile.am | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/Makefile.am b/src/Makefile.am
index 1b0d8136e..bb9461c52 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -107,12 +107,12 @@ endif
 
 DISTCLEANFILES = timestamp.h
 BUILT_SOURCES = timestamp.h
-git_current := $(shell cut -c6- $(top_srcdir)/.git/HEAD)
-timestamp.h: $(top_srcdir)/.git/${git_current} Makefile
+.PHONY: timestamp.h
+timestamp.h: Makefile
 	@echo Making $@
 	@echo "#ifndef GNUPLOT_TIMEBASE_H_INCLUDED" >$@t
 	@echo "#define GNUPLOT_TIMEBASE_H_INCLUDED" >>$@t
-	@echo "const char gnuplot_date[] = \"`git --git-dir '$(top_srcdir)/.git' log -1 --format=%ci | cut -b-11`\";" >>$@t
+	@echo "const char gnuplot_date[] = \"`git --git-dir '$(top_srcdir)/.git' log -1 --format=%cs || stat -c '%.10y' Makefile.am`\";" >>$@t
 	@echo "#endif /* GNUPLOT_TIMEBASE_H_INCLUDED */" >> $@t
 	@if cmp -s $@ $@t; then rm -f $@t; else mv $@t $@; fi
 
-- 
2.27.0