deps: build libslvs as a dependency instead of vendoring it in src

Review request on PR #15238: "Move the SLVS to deps if the source code remains
unmodified. If any changes were made to the source code, move it to deps_src."

It is unmodified — all 20 files under src/libslic3r/slvs were byte-identical to
JacobStoren/SolveSpaceLib@4d87045, the extraction of solvespace.com's libslvs.
So deps/ it is, fetched by hash like every other dependency.

Only the CMakeLists is ours: upstream's builds a demo executable and installs
nothing, so deps/SLVS/CMakeLists.txt.in replaces it via PATCH_COMMAND — the same
shape deps/OpenCSG already uses. The public header keeps its spelling, so
SketchSolver.cpp still says `#include <slvs.h>` and needs no edit.

The CI deps cache is keyed on hashFiles('deps/**'), so it rebuilds itself.

Verified: dep_SLVS builds and installs, libslic3r links against SLVS::slvs, and
the CAD suite is unchanged at 2518 assertions in 194 test cases.

deps/CMakeLists.txt

@@ -350,6 +350,7 @@ include(GLEW/GLEW.cmake)
 
 include(GLFW/GLFW.cmake)
 include(OpenCSG/OpenCSG.cmake)
+include(SLVS/SLVS.cmake)
 
 include(TBB/TBB.cmake)
 
@@ -436,6 +437,7 @@ set(_dep_list
     dep_NLopt
     dep_OpenVDB
     dep_OpenCSG
+    dep_SLVS
     dep_OpenCV
     dep_Eigen
     dep_CGAL