textproc/libxslt: switch back to autotools

Many ports use xsltproc as a BUILD_DEPENDS. CMake itself unconditionally
depends on ftp/curl, for which certain options can pull in dependencies
using xsltproc to build.

While here:

• DIST_SUBDIR to gnome
• add devel/icu LIB_DEPENDS

PR: 262853
Pointy-hat to: diizzy
Approved by: fluffy (mentor)
Differential Revision: https://reviews.freebsd.org/D34705