RFR: JDK-8144039: Enable javac server by default

[Erik Joelsson]

Hello,

The --enable-javac-server feature seems to be working well and is 
providing considerable speedup when building the JDK. We should make the 
option enabled by default.

As an example, on my local machine, a run of "make exploded-image" 
without javac server took 00:04:09 and with javac server 00:03:20. On 
machines with less cpus I have seen even bigger improvements.

Bug: https://bugs.openjdk.java.net/browse/JDK-8144039
Patch:
diff -r 358fb90ae6e6 common/autoconf/build-performance.m4
--- a/common/autoconf/build-performance.m4
+++ b/common/autoconf/build-performance.m4
@@ -413,7 +413,7 @@

    AC_ARG_ENABLE([javac-server], [AS_HELP_STRING([--enable-javac-server],
        [use only the server part of sjavac for faster javac compiles 
@<:@disabled@:>@])],
-      [ENABLE_JAVAC_SERVER="${enableval}"], [ENABLE_JAVAC_SERVER="no"])
+      [ENABLE_JAVAC_SERVER="${enableval}"], [ENABLE_JAVAC_SERVER="yes"])
    if test "x$JVM_ARG_OK" = "xfalse"; then
      AC_MSG_WARN([Could not set -Xms${MS_VALUE}M -Xmx${MX_VALUE}M, 
disabling javac server])
      ENABLE_JAVAC_SERVER="no"
diff -r 358fb90ae6e6 common/autoconf/generated-configure.sh

/Erik