Changes needed to compile JDK 8 on MacOS with clang compiler

It is preferred to build with clang on MacOS, with the fallback of using gcc.

Something along these lines:

% hg diff common/autoconf/toolchain.m4
diff --git a/common/autoconf/toolchain.m4 b/common/autoconf/toolchain.m4
--- a/common/autoconf/toolchain.m4
+++ b/common/autoconf/toolchain.m4
@@ -60,7 +60,20 @@ AC_DEFUN([TOOLCHAIN_CHECK_COMPILER_VERSI
         AC_MSG_ERROR([Target CPU mismatch. We are building for $OPENJDK_TARGET_CPU but CL is for "$COMPILER_CPU_TEST"; expected "x64".])
       fi
     fi
+ elif test "x$OPENJDK_TARGET_OS" = xmacosx; then
+ COMPILER_VERSION_TEST=`$COMPILER --version 2>&1 | $HEAD -n 1`
+ # Check that this is likely to be clang|GCC.
+ $COMPILER --version 2>&1 | $EGREP "Apple clang version|Free Software Foundation" > /dev/null
+ if test $? -ne 0; then
+ AC_MSG_NOTICE([The $COMPILER_NAME compiler (located as $COMPILER) does not seem to be the required compiler.])
+ AC_MSG_NOTICE([The result from running with --version was: "$COMPILER_VERSION_TEST"])
+ AC_MSG_ERROR([clang or GCC compiler is required. Try setting --with-tools-dir.])
+ fi
+ # First line typically looks something like:
+ COMPILER_VERSION=`$ECHO $COMPILER_VERSION_TEST | $SED -n "s/^.* \(@<:@1-9@:>@@<:@0-9.@:>@*\)/\1/p"`
+ COMPILER_VENDOR=`$ECHO $COMPILER_VERSION_TEST | $SED -n "s/^\(.*\) @<:@1-9@:>@@<:@0-9.@:>@*/\1/p"`
   else
+ # else must be l*ux and therefore gcc.
     COMPILER_VERSION_TEST=`$COMPILER --version 2>&1 | $HEAD -n 1`
     # Check that this is likely to be GCC.
     $COMPILER --version 2>&1 | $GREP "Free Software Foundation" > /dev/null
@@ -242,6 +255,7 @@ fi
 
 # On windows, only cl.exe is supported.
 # On Solaris, cc is preferred to gcc.
+# On MacOS, clang is preferred to gcc.
 # Elsewhere, gcc is preferred to cc.
 
 if test "x$CC" != x; then
@@ -250,6 +264,8 @@ elif test "x$OPENJDK_TARGET_OS" = "xwind
   COMPILER_CHECK_LIST="cl"
 elif test "x$OPENJDK_TARGET_OS" = "xsolaris"; then
   COMPILER_CHECK_LIST="cc gcc"
+elif test "x$OPENJDK_TARGET_OS" = "xmacosx"; then
+ COMPILER_CHECK_LIST="clang gcc cc"
 else
   COMPILER_CHECK_LIST="gcc cc"
 fi
@@ -510,8 +526,7 @@ if test "x$GCC" = xyes; then
 
     # Linking is different on MacOSX
     if test "x$OPENJDK_TARGET_OS" = xmacosx; then
- # Might change in the future to clang.
- COMPILER_NAME=gcc
+ COMPILER_NAME=clang
         SHARED_LIBRARY='lib[$]1.dylib'
         SHARED_LIBRARY_FLAGS="-dynamiclib -compatibility_version 1.0.0 -current_version 1.0.0 $PICFLAG"
         SHARED_LIBRARY_SUFFIX='.dylib'