Fix linux/debian/rules for building on Ubuntu 20.04

linux/debian/rules

@@ -5,10 +5,18 @@ DEB_TARGET_GNU_TYPE := $(shell dpkg-architecture -qDEB_TARGET_GNU_TYPE)
 DEB_HOST_GNU_TYPE := $(shell dpkg-architecture -qDEB_HOST_GNU_TYPE)
 DEB_BUILD_GNU_TYPE := $(shell dpkg-architecture -qDEB_BUILD_GNU_TYPE)
 
+# DEB_BUILD_GNU_TYPE is the system running the build (this system)
+# DEB_HOST_GNU_TYPE is the system jamulus is being built for (the target)
+# DEB_TARGET_GNU_TYPE is only relevant when building cross-compilers
+$(info DEB_BUILD_GNU_TYPE=$(DEB_BUILD_GNU_TYPE) DEB_HOST_GNU_TYPE=$(DEB_HOST_GNU_TYPE) DEB_TARGET_GNU_TYPE=$(DEB_TARGET_GNU_TYPE))
+
 ifeq ($(DEB_BUILD_GNU_TYPE),$(DEB_HOST_GNU_TYPE))
 	QMAKE := qmake
 else
-	QMAKE := "/usr/lib/$(DEB_BUILD_GNU_TYPE)/qt5/bin/qmake" -qtconf "/usr/lib/$(DEB_TARGET_GNU_TYPE)/qt5/qt.conf" -spec "/usr/lib/$(DEB_TARGET_GNU_TYPE)/qt5/mkspecs/$(DEB_TARGET_GNU_TYPE)-g++" LIBS+="-lstdc++ -lm"
+	# For an unknown reason, when called from this makefile in Ubuntu 20.04,
+	# qmake needs an option between qmake and -qtconf, else it gives an error.
+	# We use -makefile, which is the default mode anyway, and this avoids the issue.
+	QMAKE := qmake -makefile -qtconf "/usr/lib/$(DEB_HOST_GNU_TYPE)/qt5/qt.conf" -spec "/usr/lib/$(DEB_HOST_GNU_TYPE)/qt5/mkspecs/$(DEB_HOST_GNU_TYPE)-g++" LIBS+="-lstdc++ -lm"
 endif
 
 %: