Fix build when nlohmann_json is not CMake-installed

When nlohmann_json is installed in other ways rather
than by using the CMake setup included within the
package, no CMake config file is installed for
nlohmann_json; and so, it is not possible to use
find_package( nlohmann_json ) in the CMake setup of
our package. This commit fixes the build for the case,
for example, when nohmann_json is installed
by simply copying the single-header file into a system
include folder as '/usr/local/include'.

GeoModelJSONParser/CMakeLists.txt

@@ -6,7 +6,12 @@ file( GLOB HEADERS GeoModelJSONParser/*.h )
 
 # Create the library.
 add_library( GeoModelJSONParser SHARED ${HEADERS} ${SOURCES} )
-target_link_libraries( GeoModelJSONParser PUBLIC nlohmann_json::nlohmann_json )
+if (nlohmann_json_FOUND)
+  # we link to nlohmann_json only  if we use a version of nlohmann_json installed through CMake;
+  # this is not needed if the single-header library is installed in a regular
+  # system include folder (e.g., '/usr/local/include')
+  target_link_libraries( GeoModelJSONParser PUBLIC nlohmann_json::nlohmann_json )
+endif()
 target_include_directories( GeoModelJSONParser PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}> )

cmake/SetupJSON.cmake

@@ -13,7 +13,7 @@ option( GEOMODEL_USE_BUILTIN_JSON
 if( GEOMODEL_USE_BUILTIN_JSON )
 
    # Tell the user what's happening.
-   message( STATUS "Building nlohmann_json as part of the project" )
+   message( STATUS "'GEOMODEL_USE_BUILTIN_JSON' was set to 'true' ==> Building nlohmann_json as part of the project" )
 
    # The include directory and library that will be produced.
    set( nlohmann_json_INCLUDE_DIR
@@ -50,6 +50,9 @@ if( GEOMODEL_USE_BUILTIN_JSON )
 else()
 
    # Just find an existing installation of nlohmann_json.
-   find_package( nlohmann_json )
+   find_package( nlohmann_json QUIET)
+   if( NOT nlohmann_json_FOUND )
+     message(STATUS "WARNING! 'nlohmann_json' was not found by CMake!! However, if you installed this single-header library in a standard system include dir (e.g., '/usr/local/include'), I will be able to use it.")
+   endif()
 
 endif()