INSTALL.md: Document the BUILD_YEAR2038 environment variable

[skip ci]

Author: fxlb

INSTALL.md

@@ -37,6 +37,10 @@ the following steps:
 run `./autogen.sh` (a shell script). The autogen.sh script will
 build the `configure` and `config.h.in` files.
 
+* If you build on a Linux 32-bit system, with Autoconf version >= 2.72
+and GNU C Library version >= 2.34, run `export BUILD_YEAR2038=yes`
+before running `./autogen.sh` to build with 64-bit time_t (Y2038-safe).
+
 On some system, you may need to set the `AUTORECONF` variable, like:
 `AUTORECONF=autoreconf-2.69 ./autogen.sh`
 to select the `autoreconf` version you want to use.