add a CMake finder for libigl

cmake/FindLIBIGL.cmake

+if(LIBIGL_INCLUDE_DIR)
+    set(LIBIGL_FOUND TRUE)
+    set(LIBIGL_INCLUDE_DIRS "${LIBIGL_INCLUDE_DIR}")
+else()
+    find_path(LIBIGL_INCLUDE_DIR
+        NAMES igl/igl_inline.h
+        PATHS "${CMAKE_SOURCE_DIR}/libs/libigl/include")
+
+    set(LIBIGL_INCLUDE_DIRS "${LIBIGL_INCLUDE_DIR}")
+
+    include(FindPackageHandleStandardArgs)
+    find_package_handle_standard_args(LIBIGL DEFAULT_MSG LIBIGL_INCLUDE_DIR)
+
+    mark_as_advanced(LIBIGL_INCLUDE_DIR)
+endif()