RFR: JDK-8213239 Configure cannot handle command overrides with arguments

[Magnus Ihse Bursie]

When overriding a tool for configure, any options passed to that tool is 
lost. For instance, NICE="nice -n0" would result in NICE being just "nice".

With this fix, if you send in an override like NICE="/usr/bin/nice -n0", 
it will verify that /usr/bin/nice exists and is executable (as before), 
and then it will use NICE verbatim. For an override like NICE="nice -n0" 
without a path, we will search the path for "nice" (just like with no 
override at all), and then we will append the "-n0" arguments to it. 
This allows for users to add arguments to tools, but still let configure 
find them using standard methods.

Bug: https://bugs.openjdk.java.net/browse/JDK-8213239
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8213239-handle-configure-overrides-with-arguments/webrev.01

/Magnus