Avoid GCC 7 to compile with

This is mainly for Ubuntu bionic, but a general dependency approach.

debian/control

@@ -11,7 +11,7 @@ Build-Depends:
   dh-systemd (>= 1.5),
   libsystemd-dev,
   flex,
-  g++ (>= 1.96),
+  g++ (>= 4:8.0) | g++-8 | g++ (<< 4:7),
   libboost-dev (>= 1.66) | libboost1.67-dev | libboost1.67-icinga-dev,
   libboost-context-dev (>= 1.66) | libboost-context1.67-dev | libboost-context1.67-icinga-dev,
   libboost-coroutine-dev (>= 1.66) | libboost-coroutine1.67-dev | libboost-coroutine1.67-icinga-dev,

debian/rules

@@ -2,6 +2,11 @@
 
 #export DH_VERBOSE=1
 
+CXX_INSTALLED_VERSION := $(shell dpkg-query --showformat='$${Version}' --show g++)
+CXX_PREFERED_VERSION = 4:8.0
+CXX_PREFERED_COMPILER = g++-8
+C_PREFERED_COMPILER = gcc-8
+
 CMAKE_FLAGS = -DICINGA2_LTO_BUILD=On
 
 DEB_HOST_ARCH = $(shell dpkg-architecture -qDEB_HOST_ARCH)
@@ -26,6 +31,17 @@ CMAKE_FLAGS += -DICINGA2_UNITY_BUILD=Off
 CMAKE_FLAGS += -DICINGA2_LTO_BUILD=Off
 endif
 
+# check if the installed default g++ is already newer as preferred
+ifneq ($(shell dpkg --compare-versions "$(CXX_INSTALLED_VERSION)" ge "$(CXX_PREFERED_VERSION)" && echo 1 || echo 0),1)
+# if not choose the prefered compiler directly
+ifneq (,$(shell command -v $(CXX_PREFERED_COMPILER) 2>/dev/null))
+# if it is installed
+$(warning Installed GCC $(CXX_INSTALLED_VERSION) is older than $(CXX_PREFERED_VERSION) - using alternative binary $(CXX_PREFERED_COMPILER))
+CMAKE_FLAGS += -DCMAKE_CXX_COMPILER=$(CXX_PREFERED_COMPILER)
+CMAKE_FLAGS += -DCMAKE_C_COMPILER=$(C_PREFERED_COMPILER)
+endif
+endif
+
 %:
 	dh $@ --with systemd --with bash_completion