gnu: Add c++-gsl.

* gnu/packages/cpp.scm (c++-gsl): New variable.
* gnu/packages/patches/c++-gsl-find-system-gtest.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Leo Prikler <leo.prikler@student.tugraz.at>

gnu/local.mk

@@ -913,6 +913,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/cursynth-wave-rand.patch			\
   %D%/packages/patches/cvs-CVE-2017-12836.patch		\
   %D%/packages/patches/cyrus-sasl-ac-try-run-fix.patch		\
+  %D%/packages/patches/c++-gsl-find-system-gtest.patch		\
   %D%/packages/patches/date-output-pkg-config-files.patch	\
   %D%/packages/patches/datefudge-gettimeofday.patch		\
   %D%/packages/patches/dbacl-include-locale.h.patch		\

gnu/packages/cpp.scm

@@ -63,6 +63,35 @@
   #:use-module (gnu packages tls)
   #:use-module (gnu packages web))
 
+(define-public c++-gsl
+  (package
+    (name "c++-gsl")
+    (version "3.1.0")
+    (source
+     (origin
+       (method git-fetch)
+       (uri
+        (git-reference
+         (url "https://github.com/microsoft/GSL.git")
+         (commit
+          (string-append "v" version))))
+       (file-name
+        (git-file-name name version))
+       (patches
+        (search-patches
+         "c++-gsl-find-system-gtest.patch"))
+       (sha256
+        (base32 "0gbvr48f03830g3154bjhw92b8ggmg6wwh5xyb8nppk9v6w752l0"))))
+    (build-system cmake-build-system)
+    (native-inputs
+     `(("googletest" ,googletest)
+       ("pkg-config" ,pkg-config)))
+    (synopsis "Guidelines Support Library")
+    (description "c++-gsl contains functions and types that are suggested for
+use by the C++ Core Guidelines maintained by the Standard C++ Foundation.")
+    (home-page "https://github.com/microsoft/GSL/")
+    (license license:expat)))
+
 (define-public libzen
   (package
     (name "libzen")

gnu/packages/patches/c++-gsl-find-system-gtest.patch

@@ -0,0 +1,96 @@
+From f5cf01083baf7e8dc8318db3648bc6098dc32d67 Mon Sep 17 00:00:00 2001
+From: Nicholas Guriev <guriev-ns@ya.ru>
+Date: Sat, 18 Apr 2020 13:30:17 +0300
+Subject: [PATCH] Search for GoogleTest via pkg-config first
+
+---
+ tests/CMakeLists.txt | 55 ++++++++++++++++++++++++--------------------
+ 1 file changed, 30 insertions(+), 25 deletions(-)
+
+diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
+index 02193197..53d475c2 100644
+--- a/tests/CMakeLists.txt
++++ b/tests/CMakeLists.txt
+@@ -1,36 +1,41 @@
+ cmake_minimum_required(VERSION 3.0.2)
+ 
+ project(GSLTests CXX)
++include(FindPkgConfig)
+ 
+ # will make visual studio generated project group files
+ set_property(GLOBAL PROPERTY USE_FOLDERS ON)
+ 
+-configure_file(CMakeLists.txt.in googletest-download/CMakeLists.txt)
+-execute_process(
+-    COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
+-    RESULT_VARIABLE result
+-    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
+-)
+-if(result)
+-    message(FATAL_ERROR "CMake step for googletest failed: ${result}")
+-endif()
++pkg_search_module(GTestMain gtest_main)
++if (NOT GTestMain_FOUND)
++    configure_file(CMakeLists.txt.in googletest-download/CMakeLists.txt)
++    execute_process(
++        COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
++        RESULT_VARIABLE result
++        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
++    )
++    if(result)
++        message(FATAL_ERROR "CMake step for googletest failed: ${result}")
++    endif()
+ 
+-execute_process(
+-    COMMAND ${CMAKE_COMMAND} --build .
+-    RESULT_VARIABLE result
+-    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
+-)
+-if(result)
+-    message(FATAL_ERROR "CMake step for googletest failed: ${result}")
+-endif()
++    execute_process(
++        COMMAND ${CMAKE_COMMAND} --build .
++        RESULT_VARIABLE result
++        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
++    )
++    if(result)
++        message(FATAL_ERROR "CMake step for googletest failed: ${result}")
++    endif()
+ 
+-set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
++    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
++    set(GTestMain_LIBRARIES gtest_main)
+ 
+-add_subdirectory(
+-    ${CMAKE_CURRENT_BINARY_DIR}/googletest-src
+-    ${CMAKE_CURRENT_BINARY_DIR}/googletest-build
+-    EXCLUDE_FROM_ALL
+-)
++    add_subdirectory(
++        ${CMAKE_CURRENT_BINARY_DIR}/googletest-src
++        ${CMAKE_CURRENT_BINARY_DIR}/googletest-build
++        EXCLUDE_FROM_ALL
++    )
++endif()
+ 
+ if (MSVC AND (GSL_CXX_STANDARD EQUAL 17))
+     set(GSL_CPLUSPLUS_OPT -Zc:__cplusplus -permissive-)
+@@ -149,7 +154,7 @@ function(add_gsl_test name)
+     target_link_libraries(${name}
+         GSL
+         gsl_tests_config
+-        gtest_main
++        ${GTestMain_LIBRARIES}
+     )
+     add_test(
+         ${name}
+@@ -254,7 +259,7 @@ function(add_gsl_test_noexcept name)
+     target_link_libraries(${name}
+         GSL
+         gsl_tests_config_noexcept
+-        gtest_main
++        ${GTestMain_LIBRARIES}
+     )
+     add_test(
+       ${name}