[m-rev.] diff: strengthen GCC version deteciotn in the configure script

[Julien Fischer]

Branches: 20.01, master

My recent fix to Mantis bug #492 doesn't work on Fedora 31, due to
its GCC being configured only return the major version when invoked
with -dumpversion.

------------------------------

Strengthen GCC version detection in the configure script.

Use "gcc -dumpfullversion" to determine the GCC version where available.
Unlike -dumpversion, its output is guaranteed to be in the form
major.minor.patch.

Include the GCC version in the configure progress message.

m4/mercury.m4:
     As above.

Julien.

diff --git a/m4/mercury.m4 b/m4/mercury.m4
index d3f8b49..8b29907 100644
--- a/m4/mercury.m4
+++ b/m4/mercury.m4
@@ -685,11 +685,22 @@ AC_SUBST([ERL])

  AC_DEFUN([MERCURY_GCC_VERSION], [
  AC_REQUIRE([AC_PROG_CC])
+ 
+AC_MSG_CHECKING([what the gcc version is])
+
+# -dumpfullversion has only been supported since GCC 7; if it is available we
+# prefer its output since it is guaranteed to be in the form major.minor.patch.
+#
+raw_gcc_version=`$CC -dumpfullversion 2> /dev/null`
+if test $? -ne 0
+then
+    raw_gcc_version=`$CC -dumpversion`
+fi

  # We expect that the major and minor version numbers will always be present.
  # MinGW-w64 may add a suffix "-win32" or "-posix" after the second or third
  # number that should be ignored.
-mercury_cv_gcc_version=`$CC -dumpversion | tr .- '  ' | {
+mercury_cv_gcc_version=`echo $raw_gcc_version | tr .- '  ' | {
      read major minor third ignore
      case $third in
          [[0-9]]*)
@@ -701,6 +712,8 @@ mercury_cv_gcc_version=`$CC -dumpversion | tr .- '  ' | {
      esac
      echo ${major:-u}_${minor:-u}_${patchlevel:-u}
      }`
+
+AC_MSG_RESULT([$mercury_cv_gcc_version])
  ])

  #-----------------------------------------------------------------------------#