Update internal nlohmann/json to 3.9.1, and add a CMake option to be able to use external nlohmann/json

.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
+    sqlite3 libsqlite3-dev libtiff-dev libcurl4-openssl-dev jq python3-pip nlohmann-json3-dev
 
 python3 -m pip install --user jsonschema
 

CMakeLists.txt

@@ -121,9 +121,49 @@ include(ProjConfig)
 include(ProjMac)
 include(policies)
 
+################################################################################
+# Check for nlohmann_json
+################################################################################
+
+set(NLOHMANN_JSON_ORIGIN "auto" CACHE STRING
+"nlohmann/json origin. The default auto will try to use external \
+nlohmann/json if possible")
+set_property(CACHE NLOHMANN_JSON_ORIGIN PROPERTY STRINGS auto internal external)
+
+# Probably not the strictest minimum, but known to work with it
+set(MIN_NLOHMANN_JSON_VERSION 3.7.0)
+
+if(NLOHMANN_JSON_ORIGIN STREQUAL "external")
+  find_package(nlohmann_json REQUIRED)
+  set(NLOHMANN_JSON "external")
+elseif(NLOHMANN_JSON_ORIGIN STREQUAL "internal")
+  set(NLOHMANN_JSON "internal")
+else()
+  find_package(nlohmann_json QUIET)
+  if(nlohmann_json_FOUND)
+    set(NLOHMANN_JSON "external")
+  else()
+    set(NLOHMANN_JSON "internal")
+  endif()
+endif()
+
+if(NLOHMANN_JSON STREQUAL "external")
+  # Check minimum version
+  if(nlohmann_json_VERSION VERSION_LESS MIN_NLOHMANN_JSON_VERSION)
+    message(STATUS "external nlohmann/json version ${nlohmann_json_VERSION} "
+      "is older than minimum requirement ${MIN_NLOHMANN_JSON_VERSION}")
+    set(NLOHMANN_JSON "internal")
+  else()
+    message(STATUS "found nlohmann/json version ${nlohmann_json_VERSION}")
+  endif()
+endif()
+
+message(STATUS "nlohmann/json: ${NLOHMANN_JSON}")
+
 ################################################################################
 # Check for sqlite3
 ################################################################################
+
 find_program(EXE_SQLITE3 sqlite3)
 if(NOT EXE_SQLITE3)
   message(SEND_ERROR "sqlite3 binary not found!")

configure.ac

@@ -358,8 +358,10 @@ dnl ---------------------------------------------------------------------------
 dnl Generate files
 dnl ---------------------------------------------------------------------------
 
-AC_CONFIG_FILES([Makefile cmake/Makefile src/Makefile include/Makefile include/proj/Makefile include/proj/internal/Makefile
-                 include/proj/internal/nlohmann/Makefile
+AC_CONFIG_FILES([Makefile cmake/Makefile src/Makefile include/Makefile include/proj/Makefile
+                 include/proj/internal/Makefile
+                 include/proj/internal/vendor/Makefile
+                 include/proj/internal/vendor/nlohmann/Makefile
                  test/Makefile test/cli/Makefile test/gie/Makefile test/gigs/Makefile test/unit/Makefile
                  man/Makefile man/man1/Makefile data/Makefile])
 if ! test "x$with_external_gtest" = "xyes" ; then

include/proj/internal/Makefile.am

@@ -1,4 +1,4 @@
-SUBDIRS = nlohmann
+SUBDIRS = vendor
 
 noinst_HEADERS = \
 	coordinatesystem_internal.hpp \

include/proj/internal/include_nlohmann_json.hpp

@@ -26,13 +26,26 @@
  * DEALINGS IN THE SOFTWARE.
  ****************************************************************************/
 
+#ifndef INCLUDE_NLOHMANN_JSON_HPP
+#define INCLUDE_NLOHMANN_JSON_HPP
+
 #if defined(__GNUC__)
 #pragma GCC system_header
 #endif
 
+#ifdef EXTERNAL_NLOHMANN_JSON
+
+#include <nlohmann/json.hpp>
+
+#else // !EXTERNAL_NLOHMANN_JSON
+
 // to avoid any clash if PROJ users have another version of nlohmann/json.hpp
 #define nlohmann proj_nlohmann
 
 #if !defined(DOXYGEN_ENABLED)
-#include "nlohmann/json.hpp"
+#include "vendor/nlohmann/json.hpp"
 #endif
+
+#endif // EXTERNAL_NLOHMANN_JSON
+
+#endif // INCLUDE_NLOHMANN_JSON_HPP

include/proj/internal/vendor/Makefile.am

@@ -0,0 +1 @@
+SUBDIRS = nlohmann