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/SLVS/CMakeLists.txt.in

@@ -0,0 +1,65 @@
+# Replaces the upstream SolveSpaceLib CMakeLists, which builds a demo executable and
+# has no install rules. The sources themselves are used verbatim.
+cmake_minimum_required(VERSION 3.13)
+
+project(SLVS VERSION 3.0)
+
+add_library(slvs
+    libslvs/constrainteq.cpp
+    libslvs/entity.cpp
+    libslvs/expr.cpp
+    libslvs/system.cpp
+    libslvs/util.cpp
+    libslvs/platform/unixutil.cpp
+    libslvs/lib.cpp
+    libslvs/SolveSpaceSystem.cpp)
+
+target_compile_features(slvs PUBLIC cxx_std_11)
+
+# LIBRARY strips the solver core out of the SolveSpace application it was extracted from.
+target_compile_definitions(slvs PRIVATE -DLIBRARY)
+if (MSVC)
+    target_compile_definitions(slvs PRIVATE -D_CRT_SECURE_NO_WARNINGS -D_SCL_SECURE_NO_WARNINGS)
+endif ()
+
+target_include_directories(slvs
+    PUBLIC  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/libslvs/include>
+    PRIVATE ${PROJECT_SOURCE_DIR}/libslvs)
+
+# libslic3r is linked into shared targets, so this has to be position independent.
+set_target_properties(slvs PROPERTIES POSITION_INDEPENDENT_CODE ON)
+
+# 2018 code, predating the project's warning settings; it is not ours to clean up.
+if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    target_compile_options(slvs PRIVATE -w -fno-strict-aliasing)
+endif ()
+
+include(CMakePackageConfigHelpers)
+include(GNUInstallDirs)
+
+write_basic_package_version_file(
+    "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake"
+    VERSION ${PROJECT_VERSION}
+    COMPATIBILITY AnyNewerVersion)
+
+install(TARGETS slvs
+    EXPORT ${PROJECT_NAME}Targets
+    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+set(ConfigPackageLocation ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME})
+
+install(EXPORT ${PROJECT_NAME}Targets
+    FILE "${PROJECT_NAME}Config.cmake"
+    NAMESPACE ${PROJECT_NAME}::
+    DESTINATION ${ConfigPackageLocation})
+
+install(FILES
+        ${PROJECT_SOURCE_DIR}/libslvs/include/slvs.h
+        ${PROJECT_SOURCE_DIR}/libslvs/include/SolveSpaceSystem.h
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+install(FILES "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake"
+    DESTINATION ${ConfigPackageLocation})

deps/SLVS/SLVS.cmake

@@ -0,0 +1,13 @@
+# libslvs — the geometric constraint solver behind the Design tab's sketch constraints.
+# Extraction of solvespace.com's libslvs, taken verbatim from JacobStoren/SolveSpaceLib;
+# only the CMakeLists is ours, because upstream's builds a demo and installs nothing.
+# GPLv3, compatible with this fork's licence. Self-contained: no external dependencies.
+orcaslicer_add_cmake_project(SLVS
+    URL https://github.com/JacobStoren/SolveSpaceLib/archive/4d8704523e4bf212fadf5189f92484244f670fea.zip
+    URL_HASH SHA256=1c4bdde9c3c6ef20ea4b50b73601de56769f2eb131b36927d7c6489f102e6c30
+    PATCH_COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_LIST_DIR}/CMakeLists.txt.in ./CMakeLists.txt
+)
+
+if (MSVC)
+    add_debug_dep(dep_SLVS)
+endif ()