RFR: JDK-8157623 Unexpected warning about ignoring value of CCACHE from environment

[Magnus Ihse Bursie]

When using --enable-ccache, we always get this warning:

configure: WARNING: Ignoring value of CCACHE from the environment. Use 
command line variables instead.

This is caused by us always setting CCACHE to an empty value. This also 
had the side effect of overriding any user-provided value. This fix 
addresses both this issues.

Bug: https://bugs.openjdk.java.net/browse/JDK-8157623
Patch inline:

diff --git a/common/autoconf/build-performance.m4 
b/common/autoconf/build-performance.m4
--- a/common/autoconf/build-performance.m4
+++ b/common/autoconf/build-performance.m4
@@ -168,7 +168,6 @@
        [AS_HELP_STRING([--enable-ccache],
        [enable using ccache to speed up recompilations @<:@disabled@:>@])])

-  CCACHE=
    CCACHE_STATUS=
    AC_MSG_CHECKING([is ccache enabled])
    if test "x$enable_ccache" = xyes; then


/Magnus