Support $SOURCE_DATE_EPOCH for reproducible man pages

The environment variable SOURCE_DATE_EPOCH [0] is standardized and can
be used to produce reproducible output. Distributions like Debian will
set this variable before the build and scdoc should use it (instead of
the current date) for any timestamps within the man pages.

[0]: https://reproducible-builds.org/docs/source-date-epoch/

src/main.c

@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE
 #include <assert.h>
 #include <ctype.h>
 #include <stdbool.h>
@@ -67,10 +68,22 @@ 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;
+		char *ret = strptime(source_date_epoch, "%s", &source_date_epoch_tm);
+		if (ret == NULL || *ret != '\0') {
+			fprintf(stderr,
+					"Error: $SOURCE_DATE_EPOCH is set but malformed.\n");
+			exit(1);
+		}
+		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 == '.') {
 			int ret = str_append_ch(name, ch);

test/preamble

@@ -37,6 +37,9 @@ test(8)
 EOF
 end 0
 
+# Make sure SOURCE_DATE_EPOCH is not set for the next tests
+unset SOURCE_DATE_EPOCH
+
 begin "Writes the appropriate header"
 scdoc <<EOF | grep '^\.TH "test" "8" "'"$(date +'%F')"'"' >/dev/null
 test(8)
@@ -66,3 +69,11 @@ scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'" "" "Header"' >/d
 test-manual(8) "" "Header"
 EOF
 end 0
+
+export SOURCE_DATE_EPOCH=$(date --date="2017-12-09 23:18:57 -0500" +'%s')
+
+begin "Supports \$SOURCE_DATE_EPOCH"
+scdoc <<EOF | grep '^\.TH "reproducible-manual" "8" "2017-12-10"' >/dev/null
+reproducible-manual(8)
+EOF
+end 0