tools: gnulib: install .m4 file with gl_ prefix

tools: gnulib: install .m4 file with gl_ prefix

It was found that there is currently a conflict for the cond.m4 that
is also shipped by automake, making the gnulib one having priority causing
problem with finding AM_CONDITIONAL macro.

To handle this, install gnulib .m4 file with a gl_ prefix to the
filename.

This make sure gnulib .m4 file won't have name conflict with automake
.m4 default files permitting correct autoreconf run of any affected
package by this.

Signed-off-by: Christian Marangi <[email protected]>