From e3efedd7394a01e6c8511f6ac215e822d6fc5b62 Mon Sep 17 00:00:00 2001
From: Even Rouault <even.rouault@spatialys.com>
Date: Tue, 1 Feb 2022 15:22:26 +0100
Subject: [PATCH] Use external gtest by default when detected

---
 .github/workflows/clang_linux/start.sh |  2 +-
 test/unit/CMakeLists.txt               | 20 ++++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/.github/workflows/clang_linux/start.sh b/.github/workflows/clang_linux/start.sh
index 3cb07c44f7..b0a5ac34cc 100755
--- a/.github/workflows/clang_linux/start.sh
+++ b/.github/workflows/clang_linux/start.sh
@@ -5,7 +5,7 @@ set -e
 apt-get update -y
 DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
     sudo autoconf automake libtool clang++-10 python3-clang-10 make cmake ccache pkg-config tar zip \
-    sqlite3 libsqlite3-dev libtiff-dev libcurl4-openssl-dev jq python3-pip nlohmann-json3-dev
+    sqlite3 libsqlite3-dev libtiff-dev libcurl4-openssl-dev jq python3-pip nlohmann-json3-dev libgtest-dev
 
 python3 -m pip install --user jsonschema
 export PATH=$HOME/.local/bin:$PATH
diff --git a/test/unit/CMakeLists.txt b/test/unit/CMakeLists.txt
index eb41a6c89b..296f0bb4b2 100644
--- a/test/unit/CMakeLists.txt
+++ b/test/unit/CMakeLists.txt
@@ -1,11 +1,27 @@
 # CMake configuration for PROJ unit tests
 
-option(USE_EXTERNAL_GTEST "Compile against external GTest" OFF)
+find_package(GTest 1.8.1)
+if(GTest_FOUND)
+  option(USE_EXTERNAL_GTEST "Compile against external GTest" ON)
+else()
+  option(USE_EXTERNAL_GTEST "Compile against external GTest" OFF)
+endif()
 
 if(USE_EXTERNAL_GTEST)
 
+if(NOT GTest_FOUND)
+    message(FATAL_ERROR "External GTest >= 1.8.1 not found")
+endif()
 message(STATUS "Using external GTest")
-find_package(GTest 1.8.1 CONFIG REQUIRED)
+
+# CMake < 3.20.0 uses GTest::GTest
+# CMake >= 3.20 uses GTest::gtest, and deprecates GTest::GTest
+# so for older CMake, create an alias from GTest::GTest to GTest::gtest
+if(NOT TARGET GTest::gtest)
+    add_library(GTest::gtest INTERFACE IMPORTED)
+    set_target_properties(GTest::gtest PROPERTIES
+                          INTERFACE_LINK_LIBRARIES "GTest::GTest")
+endif()
 
 else()