Handle dates past January 19th, 2038

Try to support dates past year 2038 on systems with 64-bit time_t.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

test/data/apts-y2k38-001

@@ -0,0 +1,4 @@
+01/19/2038 [1] Event 1
+01/20/2038 [1] Event 2
+01/01/2100 [1] Event 3
+01/01/9999 [1] Event 4

test/y2k38-001.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+. "${TEST_INIT:-./test-init.sh}"
+
+if [ "$1" = 'actual' ]; then
+  "$CALCURSE" --read-only -D "$DATA_DIR"/ -c "$DATA_DIR/apts-y2k38-001" \
+    -d01/19/2038
+  "$CALCURSE" --read-only -D "$DATA_DIR"/ -c "$DATA_DIR/apts-y2k38-001" \
+    -d01/20/2038
+  "$CALCURSE" --read-only -D "$DATA_DIR"/ -c "$DATA_DIR/apts-y2k38-001" \
+    -d01/01/2100
+  "$CALCURSE" --read-only -D "$DATA_DIR"/ -c "$DATA_DIR/apts-y2k38-001" \
+    -d01/01/9999
+elif [ "$1" = 'expected' ]; then
+  cat <<EOD
+01/19/38:
+ * Event 1
+01/20/38:
+ * Event 2
+01/01/00:
+ * Event 3
+01/01/99:
+ * Event 4
+EOD
+else
+  ./run-test "$0"
+fi