Document that --enable-static-pie implies PIE

Document that --enable-static-pie implies PIE

To build static PIE, all .o files are compiled with -fPIE.  Since
--enable-static-pie is designed to provide additional security hardening
benefits, it also implies that glibc programs and tests are created as
dynamic position independent executables (PIE) by default for better
security hardening.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
	* manual/install.texi: Document that --enable-static-pie
	implies PIE.
	* INSTALL: Regenerated.