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-use-system-googletest.patch

@@ -0,0 +1,94 @@
+From 6693b9c8ff1ae1ec02c9002c0a8f5f416f0c88f0 Mon Sep 17 00:00:00 2001
+From: Maxim Cournoyer <maxim.cournoyer@gmail.com>
+Date: Sun, 12 Jan 2020 23:23:33 -0500
+Subject: [PATCH 3/3] build: Prefer googletest from the system.
+
+Fixes issue #4.
+
+The build system now look for a system installed googletest or falls
+back to the bundled copy of googletest otherwise.
+
+* CMakeLists.txt: Add call to find_package(GTest).
+(include_directories): Remove "src/googletest/googletest/include" from
+arg.
+* src/CMakeLists.txt[NOT GTEST_FOUND]: Define the gtest and gtest_main
+libraries when GTEST_FOUND is false.  Globally include the googletest
+headers here.  Define aliases for the gtest and gtest_main libraries
+that match those defined by the FindGTest module.
+* src/tests/CMakeLists.txt: Replace references to gtest_main and gtest
+by GTest::Main and GTest::GTest, respectively.
+---
+ CMakeLists.txt           |  3 ++-
+ src/CMakeLists.txt       | 15 +++++++++++----
+ src/tests/CMakeLists.txt |  6 +++---
+ 3 files changed, 16 insertions(+), 8 deletions(-)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index bfeb8f03..96fbd58c 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -25,13 +25,14 @@ find_package(DoubleConversion REQUIRED)
+ find_package(PkgConfig REQUIRED)
+ find_package(Cython)
+ find_package(Threads)
++find_package(GTest)
+ pkg_check_modules(GTK3 gtk+-3.0)
+ pkg_check_modules(GLIB glib-2.0)
+ pkg_check_modules(CAIRO cairo)
+ pkg_check_modules(GSL gsl)
+ 
+ # Add global include and link directories
+-include_directories(src src/googletest/googletest/include ${CMAKE_CURRENT_BINARY_DIR})
++include_directories(src ${CMAKE_CURRENT_BINARY_DIR})
+ link_directories(${CMAKE_CURRENT_SOURCE_DIR}/src/2geom)
+ 
+ check_cxx_source_compiles("#include <math.h>\nint main() { double a=0.5,b=0.5,c=0.5; sincos(a, &b, &c); return 0; }" HAVE_SINCOS)
+diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
+index eefb3ac5..941dc4c7 100644
+--- a/src/CMakeLists.txt
++++ b/src/CMakeLists.txt
+@@ -1,7 +1,14 @@
+-add_library(gtest SHARED googletest/googletest/src/gtest-all.cc)
+-target_include_directories(gtest PRIVATE googletest/googletest)
+-target_link_libraries(gtest Threads::Threads)
+-add_library(gtest_main STATIC googletest/googletest/src/gtest_main.cc)
++if(NOT GTEST_FOUND)
++  message("No system googletest library: using bundled copy.")
++  add_library(gtest SHARED googletest/googletest/src/gtest-all.cc)
++  add_library(gtest_main STATIC googletest/googletest/src/gtest_main.cc)
++  target_include_directories(gtest PRIVATE googletest/googletest)
++  target_link_libraries(gtest Threads::Threads)
++  include_directories(SYSTEM googletest/googletest/include)
++  # Aliases to share the same nomenclature with FindGTest.
++  add_library(GTest::GTest ALIAS gtest)
++  add_library(GTest::Main ALIAS gtest_main)
++endif()
+ 
+ add_subdirectory(2geom)
+ add_subdirectory(tests)
+diff --git a/src/tests/CMakeLists.txt b/src/tests/CMakeLists.txt
+index 626cfd87..3538f8cf 100644
+--- a/src/tests/CMakeLists.txt
++++ b/src/tests/CMakeLists.txt
+@@ -34,15 +34,15 @@ foreach(source ${2GEOM_GTESTS_SRC})
+ 	add_executable(${source} ${source}.cpp)
+ 	target_include_directories(${source} PRIVATE ${GSL_INCLUDE_DIRS}
+ 	  ${GTK3_INCLUDE_DIRS} ${GLIB_INCLUDE_DIRS})
+-	target_link_libraries(${source} 2geom gtest_main gtest ${GSL_LIBRARIES}
+-	  ${GTK3_LIBRARIES} ${GLIB_LIBRARIES})
++	target_link_libraries(${source} 2geom GTest::Main GTest::GTest
++	  ${GSL_LIBRARIES} ${GTK3_LIBRARIES} ${GLIB_LIBRARIES})
+ 	add_test(NAME ${source} COMMAND ${source})
+ endforeach()
+ 
+ foreach(source ${2GEOM_TESTS_SRC})
+     add_executable(${source} ${source}.cpp)
+     target_include_directories(${source} PRIVATE ${GSL_INCLUDE_DIRS} ${GTK3_INCLUDE_DIRS})
+-    target_link_libraries(${source} 2geom gtest ${GSL_LIBRARIES} ${GTK3_LIBRARIES})
++    target_link_libraries(${source} 2geom GTest::GTest ${GSL_LIBRARIES} ${GTK3_LIBRARIES})
+     add_test(NAME ${source} COMMAND ${source})
+ endforeach(source)
+ 
+-- 
+2.24.1
+