allow use poly2tri from system

CMakeLists.txt

@@ -217,11 +217,14 @@ if(WITH_CORE)
   endif()
 
   # try to configure and build MDAL support
-  set (WITH_INTERNAL_MDAL TRUE CACHE BOOL "Determines whether MDAL support should be built")
+  set (WITH_INTERNAL_MDAL TRUE CACHE BOOL "Determines whether MDAL should be built from internal copy")
   if (NOT WITH_INTERNAL_MDAL)
     set (MDAL_PREFIX "" CACHE PATH "Path to MDAL base directory")
   endif()
 
+  # try to configure and build POLY2TRI support
+  set (WITH_INTERNAL_POLY2TRI TRUE CACHE BOOL "Determines whether POLY2TRI should be built from internal copy")
+
   # try to configure and build POSTGRESQL support
   set (WITH_POSTGRESQL TRUE CACHE BOOL "Determines whether POSTGRESQL support should be built")
   if (WITH_POSTGRESQL)
@@ -369,6 +372,10 @@ if(WITH_CORE)
     find_package(MDAL REQUIRED) # MDAL provider
   endif()
 
+  if (NOT WITH_INTERNAL_POLY2TRI)
+    find_package(Poly2Tri REQUIRED)
+  endif()
+
   find_package(SpatiaLite REQUIRED)
 
   if (NOT PROJ_FOUND OR NOT GEOS_FOUND OR NOT GDAL_FOUND)

cmake/FindPoly2Tri.cmake

@@ -0,0 +1,24 @@
+# Find Poly2Tri
+# ~~~~~~~~~
+# Copyright (c) 2020, Peter Petrik <zilolv at gmail dot com>
+# Redistribution and use is allowed according to the terms of the BSD license.
+# For details see the accompanying COPYING-CMAKE-SCRIPTS file.
+#
+#
+# Once run this will define: 
+#  Poly2Tri_FOUND - System has Poly2Tri
+#  Poly2Tri_INCLUDE_DIR - The Poly2Tri include directory
+#  Poly2Tri_LIBRARY - The library needed to use Poly2Tri
+
+find_path(Poly2Tri_INCLUDE_DIR poly2tri.h
+          HINTS $ENV{LIB_DIR}/include)
+
+find_library(Poly2Tri_LIBRARY NAMES poly2tri libpoly2tri
+             HINTS $ENV{LIB_DIR}/lib)
+
+include(FindPackageHandleStandardArgs)
+
+find_package_handle_standard_args(Poly2Tri DEFAULT_MSG
+                                  Poly2Tri_LIBRARY Poly2Tri_INCLUDE_DIR)
+
+mark_as_advanced( Poly2Tri_INCLUDE_DIR Poly2Tri_LIBRARY )

src/core/CMakeLists.txt

@@ -11,11 +11,6 @@ set(QGIS_CORE_SRCS
   ${CMAKE_SOURCE_DIR}/external/nmea/time.c
   ${CMAKE_SOURCE_DIR}/external/nmea/tok.c
 
-  ${CMAKE_SOURCE_DIR}/external/poly2tri/common/shapes.cc
-  ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/advancing_front.cc
-  ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/cdt.cc
-  ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/sweep_context.cc
-  ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/sweep.cc
   ${CMAKE_SOURCE_DIR}/external/meshOptimizer/simplifier.cpp
 
   callouts/qgscallout.cpp
@@ -734,6 +729,19 @@ set(QGIS_CORE_SRCS
   qgsuserprofilemanager.cpp
 )
 
+if (WITH_INTERNAL_POLY2TRI)
+  set(QGIS_CORE_SRCS ${QGIS_CORE_SRCS}
+    ${CMAKE_SOURCE_DIR}/external/poly2tri/common/shapes.cc
+    ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/advancing_front.cc
+    ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/cdt.cc
+    ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/sweep_context.cc
+    ${CMAKE_SOURCE_DIR}/external/poly2tri/sweep/sweep.cc
+  )
+  include_directories(${CMAKE_SOURCE_DIR}/external/poly2tri)
+else ()
+  include_directories(${Poly2Tri_INCLUDE_DIR})
+endif ()
+
 file(GLOB JSON_HELP_FILES "${CMAKE_SOURCE_DIR}/resources/function_help/json/*")
 if(CMAKE_VERSION VERSION_LESS "3.18" AND NOT USING_NINJA)
   string(REPLACE "$" "$$" JSON_HELP_FILES "${JSON_HELP_FILES}")
@@ -1624,7 +1632,6 @@ include_directories(
   ${CMAKE_SOURCE_DIR}/external/nlohmann
   ${CMAKE_SOURCE_DIR}/external/kdbush/include
   ${CMAKE_SOURCE_DIR}/external/nmea
-  ${CMAKE_SOURCE_DIR}/external/poly2tri
   ${CMAKE_SOURCE_DIR}/external/rtree/include
   ${CMAKE_SOURCE_DIR}/external/meshOptimizer
 )
@@ -1730,6 +1737,9 @@ if(ENABLE_MODELTEST)
   target_link_libraries(qgis_core ${Qt5Test_LIBRARIES})
 endif()
 
+if (NOT WITH_INTERNAL_POLY2TRI)
+   target_link_libraries(qgis_core ${Poly2Tri_LIBRARY})
+endif()
 
 if(HAVE_OPENCL)
   target_link_libraries(qgis_core ${OpenCL_LIBRARY})