RFC: Use gcc -dumpfullversion to get minor version with gcc-7.x

In gcc-7.1, the behavior of -dumpversion changed to be configured
at compile-time. On some distributions it only dumps the major version
(e.g., Ubuntu), so the current checks for the gcc minor version report
false negatives. As a replacement, gcc-7.1 introduced -dumpfullversion
which always prints the full version.

Update the gcc version detection in Makefile.system to employ
-dumpfullversion with gcc-7 and newer.

Posting this patch for discussion, since it emerged from discussions
around issue #2668 and PR #2669. It is not solving a problem right now,
but may be useful in the future.

Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>

Makefile.system

@@ -286,8 +286,15 @@ GCCVERSIONEQ5 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` = 5)
 GCCVERSIONGT5 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \> 5)
 GCCVERSIONGTEQ7 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 7)
 GCCVERSIONGTEQ9 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 9)
-GCCMINORVERSIONGTEQ2 := $(shell expr `$(CC) -dumpversion | cut -f2 -d.` \>= 2)
-GCCMINORVERSIONGTEQ7 := $(shell expr `$(CC) -dumpversion | cut -f2 -d.` \>= 7)
+# Note that the behavior of -dumpversion is compile-time-configurable for
+# gcc-7.x and newer. Use -dumpfullversion there
+ifeq ($(GCCVERSIONGTEQ7),1)
+	GCCDUMPVERSION_PARAM := -dumpfullversion
+else
+	GCCDUMPVERSION_PARAM := -dumpversion
+endif
+GCCMINORVERSIONGTEQ2 := $(shell expr `$(CC) $(GCCDUMPVERSION_PARAM) | cut -f2 -d.` \>= 2)
+GCCMINORVERSIONGTEQ7 := $(shell expr `$(CC) $(GCCDUMPVERSION_PARAM) | cut -f2 -d.` \>= 7)
 endif
 
 #