gnu: Add lib2geom.

* gnu/packages/graphics.scm (lib2geom): New variable.
* gnu/packages/patches/lib2geom-enable-assertions.patch,
lib2geom-link-tests-against-glib.patch,
lib2geom-use-system-googletest.patch: New files.
* gnu/local.mk (dist_patch_DATA): Register new patches.

gnu/packages/patches/lib2geom-enable-assertions.patch

@@ -0,0 +1,36 @@
+From 4aa78f52232682b353eb15c219171e466987bac7 Mon Sep 17 00:00:00 2001
+From: Maxim Cournoyer <maxim.cournoyer@gmail.com>
+Date: Thu, 16 Jan 2020 00:19:29 -0500
+Subject: [PATCH] build: Enable assertions for the RelWithDebugInfo build type.
+
+This fixes issue #5 (see:
+https://gitlab.com/inkscape/lib2geom/issues/5).
+
+* CMakeLists.txt: Remove the "-DNDEBUG" CXX flag from the default
+configuration for the RelWithDebugInfo build type.
+---
+ CMakeLists.txt | 8 ++++++++
+ 1 file changed, 8 insertions(+)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index bfeb8f03..a663a1b0 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -19,6 +19,14 @@ enable_testing()
+ 
+ include(CheckCXXSourceCompiles)
+ 
++# Enable assertions for the RelWithDebugInfo build type.  This is
++# useful as some tests make use of it (see:
++# https://gitlab.com/inkscape/lib2geom/issues/5).
++if (CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
++  string(REPLACE "-DNDEBUG" ""
++    CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
++endif()
++
+ # Find dependencies
+ find_package(Boost 1.40 REQUIRED)
+ find_package(DoubleConversion REQUIRED)
+-- 
+2.24.1
+