RFR: JDK-8074859 Turn on warnings as error

[Magnus Ihse Bursie]

With JDK-8074096, the number of warnings in the product was reduced to a 
minimum. This enables the next step, which is turning on the respective 
compiler flags that turns warnings into errors. In the long run, this is 
the only way to keep the warnings from creeping back.

Even with JDK-8074096, the product does not build 100% warning free. 
This is due to some warnings that cannot be disabled, or (in one case) 
where C and C++ code is mixed, and warnings for both languages cannot be 
used. A system similar to the one introduced in JDK-8074096 is therefore 
needed, in which individual libraries can be exempted from this flag, 
until such warnings are fixed. A library can thus disable warnings as 
errors with WARNINGS_AS_ERRORS := false, or (better) use a 
toolchain-specific version, e.g WARNINGS_AS_ERRORS_gcc := false. This is 
intended as a temporary measure, though. The long-term solution is 
reasonably to fix the warnings and remove that argument.

Also, different versions of compilers can generate a different set of 
warnings. It is therefore necessary to be able to turn off this 
globally. Therefor a new flag for configure is introduced: 
--disable-warnings-as-errors.

While the code compiles without errors on the build systems used 
internally at Oracle, this might not be the case on other combinations 
of operating system versions and toolchain versions. To facilitate for 
unexpecting developers, a help message is added if the build fails, that 
suggests using --disable-warnings-as-errors. This solution was chosen as 
a compromise between the "hard core" solution of turning on warnings as 
errors by default for anyone, and the cowar... erm, conservative 
solution of checking if the compiler versions exactly match what's used 
inside Oracle (and therefore regularly tested), and only turn it on in 
that case.

Similarly to JDK-8074096, I intend to file follow-up bugs for each 
individual library that got a WARNINGS_AS_ERRORS_* := false.

Bug: https://bugs.openjdk.java.net/browse/JDK-8074859
WebRev for top: 
http://cr.openjdk.java.net/~ihse/JDK-8074859-warnings-as-errors-top/webrev.01
WebRev for jdk: 
http://cr.openjdk.java.net/~ihse/JDK-8074859-warnings-as-errors-jdk/webrev.01

Some comments:

* I needed to add a few more DISABLED_WARNINGS. For windows, this is 
most likely due to the recent compiler change. For other libraries, I'm 
not sure, but it might well be the result of recent changes that has 
introduced new warnings. If so, it highlights the need of this patch to 
keep the build warning free.

* For a few libraries and toolchains, there is *both* WARNINGS_AS_ERRORS 
:= false and a DISABLED_WARNINGS list. This is the case if not all 
warnings are possible to disable.

* I have removed some incorrect uses of SHARED_LIBRARY_FLAGS. This is 
included in our JDK LDFLAGS, so it should not be set separately, and 
definitely not as CFLAGS. (This caused compiler warnings, which now 
turned into errors.) However, a more suitable long-term solution is 
probably to move the knowledge of how to create shared libraries more 
specifically into SetupNativeCompilation, and not set it as part of the 
JDK flags.

/Magnus