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.

src/Makefile.cfg

@@ -1,3 +1,4 @@
+# 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