Make it possible to install sagelib on gentoo without patches to `config.py`

[tobiasdiez]

From https://github.com/cschwan/sage-on-gentoo/blob/master/sci-mathematics/sagemath/files/sagemath-10.8-config.py.in.patch:

• MAXIMA = "maxima -l ecl"
• MATHJAX_DIR = "@GENTOO_PORTAGE_EPREFIX@/usr/share/mathjax" -> Migrate Mathjax detection to meson #40404
• GAP_ROOT_PATHS = "@GENTOO_PORTAGE_EPREFIX@/usr/@libdir@/gap;@GENTOO_PORTAGE_EPREFIX@/usr/share/gap"
• FOURTITWO

SAGE_SHARE   = "@GENTOO_PORTAGE_EPREFIX@/usr/share/sage"
SAGE_DOC     = "@GENTOO_PORTAGE_EPREFIX@/usr/share/doc/@GENTOO_PORTAGE_PF@"

JMOL_DIR     = "@GENTOO_PORTAGE_EPREFIX@/usr/share/sage-jmol-bin/lib"
MTXLIB       = "@GENTOO_PORTAGE_EPREFIX@/usr/share/meataxe"
PPLPY_DOCS   = "@GENTOO_PORTAGE_EPREFIX@/usr/share/doc/@PPLY_DOC_VERS@/html"

LIE_INFO_DIR = "@GENTOO_PORTAGE_EPREFIX@/usr/share/lie"

[tobiasdiez]

@kiwifb why is -l ecl necessary for maxima?

[kiwifb]

@tobiasdiez I guess it is not strictly necessary and I should remove it. Maxima can be installed with several lisp interpreter and the -l switch enables you to select which one rather than what is available in pecking order (which would have sbcl first if present).
At some point enforcing ecl was desirable, but I think we have moved from that.