scdoc: Use $SOURCE_DATE_EPOCH to produce reproducible man pages

Use this patch until something like it is in the upstream repository.
Without it, the current date is used for the man pages, which makes them
non-reproducible.

pkgs/tools/typesetting/scdoc/default.nix

@@ -9,6 +9,8 @@ stdenv.mkDerivation rec {
     sha256 = "0a9sxifzsbj24kjnpc0525i91ni2vkwizhgvwx1m9shvfkiisnc6";
   };
 
+  patches = [ ./use-source-date-epoch.patch ];
+
   postPatch = ''
     substituteInPlace Makefile \
       --replace "-static" "" \

pkgs/tools/typesetting/scdoc/use-source-date-epoch.patch

@@ -0,0 +1,75 @@
+diff --git a/src/main.c b/src/main.c
+index 14b08d2..e2cc33e 100644
+--- a/src/main.c
++++ b/src/main.c
+@@ -3,6 +3,7 @@
+ #include <stdbool.h>
+ #include <stdio.h>
+ #include <stdlib.h>
++#define __USE_XOPEN
+ #include <time.h>
+ #include <unistd.h>
+ #include "string.h"
+@@ -66,10 +67,17 @@ static void parse_preamble(struct parser *p) {
+ 	int section = -1;
+ 	uint32_t ch;
+ 	char date[256];
+-	time_t now;
+-	time(&now);
+-	struct tm *now_tm = localtime(&now);
+-	strftime(date, sizeof(date), "%F", now_tm);
++	char *source_date_epoch = getenv("SOURCE_DATE_EPOCH");
++	if (source_date_epoch != NULL) {
++		struct tm source_date_epoch_tm;
++		strptime(source_date_epoch, "%s", &source_date_epoch_tm);
++		strftime(date, sizeof(date), "%F", &source_date_epoch_tm);
++	} else {
++		time_t now;
++		time(&now);
++		struct tm *now_tm = localtime(&now);
++		strftime(date, sizeof(date), "%F", now_tm);
++	}
+ 	while ((ch = parser_getch(p)) != UTF8_INVALID) {
+ 		if ((ch < 0x80 && isalnum(ch)) || ch == '_' || ch == '-' || ch == '.') {
+ 			assert(str_append_ch(name, ch) != -1);
+diff --git a/test/preamble b/test/preamble
+index 03e2d0c..eeb734b 100755
+--- a/test/preamble
++++ b/test/preamble
+@@ -38,31 +38,31 @@ EOF
+ end 0
+ 
+ begin "Writes the appropriate header"
+-scdoc <<EOF | grep '^\.TH "test" "8" "'"$(date +'%F')"'"' >/dev/null
++scdoc <<EOF | grep '^\.TH "test" "8" "'"$(date +'%F' --date=@${SOURCE_DATE_EPOCH:-$(date +'%s')})"'"' >/dev/null
+ test(8)
+ EOF
+ end 0
+ 
+ begin "Preserves dashes"
+-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'"' >/dev/null
++scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F' --date=@${SOURCE_DATE_EPOCH:-$(date +'%s')})"'"' >/dev/null
+ test-manual(8)
+ EOF
+ end 0
+ 
+ begin "Handles extra footer field"
+-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'" "Footer"' >/dev/null
++scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F' --date=@${SOURCE_DATE_EPOCH:-$(date +'%s')})"'" "Footer"' >/dev/null
+ test-manual(8) "Footer"
+ EOF
+ end 0
+ 
+ begin "Handles both extra fields"
+-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'" "Footer" "Header"' >/dev/null
++scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F' --date=@${SOURCE_DATE_EPOCH:-$(date +'%s')})"'" "Footer" "Header"' >/dev/null
+ test-manual(8) "Footer" "Header"
+ EOF
+ end 0
+ 
+ begin "Emits empty footer correctly"
+-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'" "" "Header"' >/dev/null
++scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F' --date=@${SOURCE_DATE_EPOCH:-$(date +'%s')})"'" "" "Header"' >/dev/null
+ test-manual(8) "" "Header"
+ EOF
+ end 0