DXF: initial libdxfrw import.

We had to fork libdxfrw since the upstream doesn't have a git
repository, a CMake buildsystem, and is quite buggy.

libdxfrw is also used in LibreCAD, but they just vendored
their version.

.gitmodules

@@ -8,3 +8,6 @@
 [submodule "extlib/libfreetype"]
 	path = extlib/libfreetype
 	url = http://git.sv.nongnu.org/r/freetype/freetype2.git
+[submodule "extlib/libdxfrw"]
+	path = extlib/libdxfrw
+	url = https://github.com/whitequark/libdxfrw.git

CMakeLists.txt

@@ -72,6 +72,9 @@ CHECK_INCLUDE_FILE("stdint.h" HAVE_STDINT_H)
 
 find_package(OpenGL REQUIRED)
 
+message(STATUS "Using in-tree libdxfrw")
+add_subdirectory(extlib/libdxfrw)
+
 if(WIN32)
     # We don't want any of the subprojects to install anything.
     # We don't actually install anything on Windows (so overriding
@@ -103,9 +106,9 @@ if(WIN32)
 
     if(NOT ZLIB_FOUND)
         message(STATUS "Using in-tree zlib")
-
         add_subdirectory(extlib/zlib)
 
+        message(STATUS "Using in-tree libpng")
         set(ZLIB_LIBRARY
             zlibstatic)
         set(ZLIB_INCLUDE_DIR

extlib/libdxfrw

@@ -0,0 +1 @@
+Subproject commit a6f7c0a9ff7b9dd847a1c673f7c414c3e10c4022

src/CMakeLists.txt

@@ -319,6 +319,7 @@ add_executable(solvespace WIN32 MACOSX_BUNDLE
     ${solvespace_SOURCES})
 
 target_link_libraries(solvespace
+    dxfrw
     ${OPENGL_LIBRARIES}
     ${PNG_LIBRARIES}
     ${ZLIB_LIBRARIES}