[m-rev.] for review: Make gcc and clang version checks more robust.

[Peter Wang]

{x86_64,i686}-w64-mingw32-gcc -dumpfullversion on Debian 12
reports "12-win32", which the MERCURY_GCC_VERSION macro failed
to parse, as it expected a minor version to be present.

m4/mercury.m4:
    In the MERCURY_GCC_VERSION macro:
      - simplify calling $CC -dumpfullversion or $CC -dumpversion
      - trim any suffix following a hyphen in the raw gcc version output
      - set major/minor/patchlevel fields containing anything
        other than a sequence of decimal digits to "u"
      - use $() syntax for command substitutions

    Modify the MERCURY_CLANG_VERSION macro in a similar way.
---
 m4/mercury.m4 | 77 +++++++++++++++++++++++++++++----------------------
 1 file changed, 44 insertions(+), 33 deletions(-)

diff --git a/m4/mercury.m4 b/m4/mercury.m4
index 5ce50fbfc..0a6231656 100644
--- a/m4/mercury.m4
+++ b/m4/mercury.m4
@@ -622,33 +622,34 @@ fi
 
 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
+# (It turns out that mingw32 gcc -dumpfullversion on Debian 12 reports
+# "12-win32" anyway.)
+raw_gcc_version=$("$CC" -dumpfullversion 2>/dev/null || "$CC" -dumpversion)
 
-# 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=`echo $raw_gcc_version | tr .- '  ' | {
-    read major minor third ignore
-    case $third in
-        [[0-9]]*)
-            patchlevel=$third
-            ;;
-        *)
-            patchlevel=
-            ;;
+# The major version number should always be present.
+# The minor version number and patchlevel are not always present.
+# MinGW-w64 may add a suffix "-win32" or "-posix" that should be ignored.
+mercury_cv_gcc_version=$(echo "${raw_gcc_version%-*}" | tr . ' ' | {
+    read major minor patchlevel ignore
+    case $major in
+	[[0-9]]*) ;;
+	*) major=u ;;
     esac
-    echo ${major:-u}_${minor:-u}_${patchlevel:-u}
-    }`
+    case $minor in
+	[[0-9]]*) ;;
+	*) minor=u ;;
+    esac
+    case $patchlevel in
+	[[0-9]]*) ;;
+	*) patchlevel=u ;;
+    esac
+    echo "${major}_${minor}_${patchlevel}"
+    })
 
 AC_MSG_RESULT([$mercury_cv_gcc_version])
 ])
@@ -728,21 +729,31 @@ AC_MSG_RESULT([$mercury_cv_cc_type])
 AC_DEFUN([MERCURY_CLANG_VERSION], [
 AC_REQUIRE([AC_PROG_CC])
 
-# We expect that the major and minor version numbers will always be present.
+AC_MSG_CHECKING([what the clang version is])
+
+raw_clang_version=$("$CC" -dumpversion)
+
+# The major version number should always be present.
 # For GCC we allow for a suffix after the second or third number that should
 # be ignored; it seems prudent to do the same for clang here as well.
-mercury_cv_clang_version=`$CC -dumpversion | tr .- '  ' | {
-    read major minor third ignore
-    case $third in
-        [[0-9]]*)
-            patchlevel=$third
-            ;;
-        *)
-            patchlevel=
-            ;;
+mercury_cv_clang_version=$(echo "${raw_clang_version%-*}" | tr . ' ' | {
+    read major minor patchlevel ignore
+    case $major in
+	[[0-9]]*) ;;
+	*) major=u ;;
     esac
-    echo ${major:-u}_${minor:-u}_${patchlevel:-u}
-    }`
+    case $minor in
+	[[0-9]]*) ;;
+	*) minor=u ;;
+    esac
+    case $patchlevel in
+	[[0-9]]*) ;;
+	*) patchlevel=u ;;
+    esac
+    echo "${major}_${minor}_${patchlevel}"
+    })
+
+AC_MSG_RESULT([$mercury_cv_clang_version])
 ])
 
 #-----------------------------------------------------------------------------#
-- 
2.42.0