debian packaging: verify absence of Qt3DExtras system headers before

falling back to internal copy

debian/control.in

@@ -423,7 +423,7 @@ Breaks: python-qgis-common (<< ${source:Version})
 Replaces: python-qgis-common (<< ${source:Version})
 Suggests:
 #eoan focal buster sid# otb-qgis,
-  saga
+ saga
 Description: Python bindings to QGIS - architecture-independent files
  QGIS is a Geographic Information System (GIS) which manages, analyzes and
  display databases of geographic information.

debian/rules

@@ -122,6 +122,8 @@ ifneq (,$(findstring $(DISTRIBUTION),"sid buster bionic eoan focal"))
 		-DGEOS_LIBRARY=/usr/lib/$(DEB_BUILD_MULTIARCH)/libgeos_c.so
 
 ifneq (,$(findstring $(DISTRIBUTION),"sid buster eoan focal"))
+$(shell test -f /usr/include/$(DEB_BUILD_MULTIARCH)/qt5/Qt3DExtras/qt3dextrasversion.h)
+ifneq (0,$(.SHELLSTATUS))
 # Qt3DExtras intentionally removed from debian (#895386) and in turn ubuntu
 	CMAKE_OPTS += \
 		-DCMAKE_PREFIX_PATH=$(realpath external/qt3dextra-headers/cmake) \
@@ -130,6 +132,8 @@ ifneq (,$(findstring $(DISTRIBUTION),"sid buster eoan focal"))
 endif
 endif
 
+endif
+
 ifneq (,$(WITH_ORACLE))
 ifeq ($(DEB_BUILD_ARCH),amd64)
 	ORACLE_INCLUDEDIR=/usr/include/oracle/18.5/client64/