Add Eigen to build.

CMakeLists.txt

@@ -57,6 +57,9 @@ set(ENABLE_OPENMP     OFF CACHE BOOL
     "Whether geometric operations will be parallelized using OpenMP")
 set(ENABLE_LTO        OFF CACHE BOOL
     "Whether interprocedural (global) optimizations are enabled")
+option(FORCE_VENDORED_Eigen3
+    "Whether we should use our bundled Eigen even in the presence of a system copy"
+    OFF)
 
 set(OPENGL 3 CACHE STRING "OpenGL version to use (one of: 1 3)")
 
@@ -190,6 +193,18 @@ set(MI_BUILD_TESTS OFF CACHE BOOL "")
 add_subdirectory(extlib/mimalloc EXCLUDE_FROM_ALL)
 set(MIMALLOC_INCLUDE_DIR ${CMAKE_SOURCE_DIR}/extlib/mimalloc/include)
 
+if(NOT FORCE_VENDORED_Eigen3)
+    find_package(Eigen3 CONFIG)
+endif()
+if(FORCE_VENDORED_Eigen3 OR NOT EIGEN3_FOUND)
+    message(STATUS "Using in-tree Eigen")
+    set(EIGEN3_FOUND             YES)
+    set(EIGEN3_INCLUDE_DIRS      ${CMAKE_SOURCE_DIR}/extlib/eigen)
+else()
+    message(STATUS "Using system Eigen: ${EIGEN3_INCLUDE_DIRS}")
+endif()
+
+
 if(WIN32 OR APPLE)
     # On Win32 and macOS we use vendored packages, since there is little to no benefit
     # to trying to find system versions. In particular, trying to link to libraries from

src/CMakeLists.txt

@@ -27,7 +27,8 @@ target_include_directories(slvs_deps INTERFACE SYSTEM
     ${PNG_PNG_INCLUDE_DIR}
     ${FREETYPE_INCLUDE_DIRS}
     ${CAIRO_INCLUDE_DIRS}
-    ${MIMALLOC_INCLUDE_DIR})
+    ${MIMALLOC_INCLUDE_DIR}
+    ${EIGEN3_INCLUDE_DIRS})
 target_link_libraries(slvs_deps INTERFACE
     dxfrw
     ${ZLIB_LIBRARY}