DOCS: update LGPL building instructions

previous build system used --enable-lgpl

Copyright

@@ -4,7 +4,7 @@ mpv as a whole is licensed under the GNU General Public License GPL version 2
 or later (called GPLv2+ in this document, see LICENSE.GPL for full license
 text) by default, or the GNU Lesser General Public License LGPL version 2 or
 later (LGPLv2.1+ in this document, see LICENSE.LGPL for full license text) if
-built with the --enable-lgpl configure switch.
+built with the -Dgpl=false configure switch.
 
 Most source files are LGPLv2.1+ or GPLv2+, but some files are available under
 more liberal licenses, such as BSD, MIT, ISC, and possibly others. Look at the
@@ -31,11 +31,11 @@ contains the complete SVN and CVS history as well.
 Some libraries are GPLv2+ or GPLv3+ only. Building mpv with Samba support makes
 it GPLv3+.
 
-mpv can be built as LGPLv2.1+ with the --enable-lgpl configure option. To add
-a LGPL mode to mpv, MPlayer code had to be relicensed from GPLv2+ to LGPLv2.1+
-by asking the MPlayer authors for permission. Since permission could not be
-obtained from everyone, LGPL mode disables the following features, some of
-them quite central:
+mpv can be built as LGPLv2.1+ with the -Dgpl=false configure option. To add a
+LGPL mode to mpv, MPlayer code had to be relicensed from GPLv2+ to LGPLv2.1+ by
+asking the MPlayer authors for permission. Since permission could not be
+obtained from everyone, LGPL mode disables the following features, some of them
+quite central:
 - Linux X11 video output
 - BSD audio output via OSS
 - NVIDIA/Linux hardware decoding (vdpau, although nvdec usually works)
@@ -45,7 +45,7 @@ Some of these will be fixed in the future. The intended use for LGPL mode is
 with libmpv, and currently it's not recommended to build mpv CLI in LGPL mode
 at all.
 
-The following files are still GPL only (--enable-lgpl disables them):
+The following files are still GPL only (-Dgpl=false disables them):
 
     audio/out/ao_jack.c             will stay GPL
     audio/out/ao_oss.c              will stay GPL

README.md

@@ -172,7 +172,7 @@ for ideas on what you could contribute with.
 
 ## License
 
-GPLv2 "or later" by default, LGPLv2.1 "or later" with `--enable-lgpl`.
+GPLv2 "or later" by default, LGPLv2.1 "or later" with `-Dgpl=false`.
 See [details.](https://github.com/mpv-player/mpv/blob/master/Copyright)
 
 ## History

libmpv/client.h

@@ -17,7 +17,7 @@
  * Note: the client API is licensed under ISC (see above) to enable
  * other wrappers outside of mpv. But keep in mind that the
  * mpv core is by default still GPLv2+ - unless built with
- * --enable-lgpl, which makes it LGPLv2+.
+ * -Dgpl=false, which makes it LGPLv2+.
  */
 
 #ifndef MPV_CLIENT_API_H_