create dummy script packages for ffmpeg and imagemagick

[seblabbe]

One advantage is that when the feature ffmpeg or imagemagick is needed, an error message will now suggest how to install ffmpeg or imagemagick according to the system (debian, cygwin, macports, homebrew, etc.).

From #32650 comment:33

See also #32926 comment:24

Part of #30930

Component: packages: optional

Author: Sébastien Labbé

Branch: 6dd8bca

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/32956

[seblabbe]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
 From [#32650 comment:33](https://github.com/sagemath/sage/issues/32650#comment:33)
+
+Part of #30930

[seblabbe]

comment:2

... and maybe also pdftocairo

[seblabbe]

Description changed:

--- 
+++ 
@@ -1,3 +1,5 @@
 From [#32650 comment:33](https://github.com/sagemath/sage/issues/32650#comment:33)
 
+See also [#32926 comment:24](https://github.com/sagemath/sage/issues/32926#comment:24)
+
 Part of #30930

[seblabbe]

Commit: 656bb52

[seblabbe]

New commits:

f94241f	32956: adding build/pkgs/ffmpeg
2f7b1cd	32956: adding build/pkgs/imagemagick
656bb52	32956: link ffmpeg and imagemagick features to the spkg

[seblabbe]

Author: Sébastien Labbé

[seblabbe]

Branch: u/slabbe/32956

[seblabbe]

comment:5

A question I have is whether ffmpeg should be removed from the following list (like it is the case for pandoc?):

$ cat build/pkgs/_recommended/distros/debian.txt 
# From https://wiki.sagemath.org/prerequisitesUbuntu

# to generate pdf documentation
texlive-latex-extra
# to convert Jupyter notebooks to pdf
texlive-xetex
# to generate pdf documentation
latexmk
# to convert Jupyter notebooks to pdf
# pandoc -- this is a separate script package
# to render text with LaTeX in Matplotlib
dvipng
# to run the Jmol 3D viewer from the console and generate images for 3D plots in the documentation
default-jdk
# to produce animations
ffmpeg
libavdevice-dev

[mkoeppe]

comment:6

Yes.

In return, add the new packages to src/doc/bootstrap in the same way that pandoc appears there

[seblabbe]

comment:7

Ok replacing the line _recommended:*|pandoc:*) by _recommended:*|pandoc:*|ffmpeg:*) ?

$ grep pandoc src/doc/bootstrap -C4
                       case "$PKG_BASE:$PKG_TYPE" in
                           *:standard)
                               SYSTEM_PACKAGES+=" $PKG_SYSTEM_PACKAGES"
                               ;;
                           _recommended:*|pandoc:*)
                               RECOMMENDED_SYSTEM_PACKAGES+=" $PKG_SYSTEM_PACKAGES"
                               ;;
                           *)
                               OPTIONAL_SYSTEM_PACKAGES+=" $PKG_SYSTEM_PACKAGES"

I find it weird that those pandoc, ffmpeg and imagemagick need to be hardcoded in the src/doc/bootstrap file. Isn't there a better way to do this?