Detect the compiler version and set the correct GCC flag

If the version is not supported by the Makefile, the flag for the latest
version supported is set instead.

(cherry picked from commit bf90fbb9)

src/Makefile.cfg

+# vim: ft=make
 #
 # Makefile.cfg for SRB2
 #
@@ -7,6 +8,42 @@
 # and other things
 #
 
+# See the following variable don't start with 'GCC'. This is
+# to avoid a false positive with the version detection...
+
+SUPPORTED_GCC_VERSIONS:=\
+	91\
+	81 82 83\
+	71 72\
+	61 62 63 64\
+	51 52 53 54\
+	40 41 42 43 44 45 46 47 48 49
+
+LATEST_GCC_VERSION=9.1
+
+# Automatically set version flag, but not if one was manually set
+ifeq   (,$(filter GCC%,$(.VARIABLES)))
+ ifneq (,$(findstring GCC,$(shell $(CC) --version))) # if it's GCC
+  version:=$(shell $(CC) -dumpversion)
+
+  # Turn version into words of major, minor
+  v:=$(subst ., ,$(version))
+  # concat. major minor
+  v:=$(word 1,$(v))$(word 2,$(v))
+
+  # If this version is not in the list, default to the latest supported
+  ifeq (,$(filter $(v),$(SUPPORTED_GCC_VERSIONS)))
+   $(info\
+		Your compiler version, GCC $(version) is not supported by the Makefile.\
+		The Makefile will assume GCC $(LATEST_GCC_VERSION).)
+   GCC$(subst .,,$(LATEST_GCC_VERSION))=1
+  else
+   $(info Detected GCC $(version) (GCC$(v)))
+   GCC$(v)=1
+  endif
+ endif
+endif
+
 ifdef GCC91
 GCC83=1
 endif