RFR: 8257913: Add more known library locations to simplify Linux cross-compilation [v3]

[Aleksey Shipilev]

On Mon, 8 Mar 2021 16:11:20 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Replace with linux with TARGET_OS
>
> make/autoconf/platform.m4 line 364:
> 
>> 362:   AC_SUBST(OPENJDK_TARGET_CPU_AUTOCONF)
>> 363:   AC_SUBST(OPENJDK_TARGET_LIBC)
>> 364:   AC_SUBST(OPENJDK_TARGET_ABI)
> 
> I realize this has already been integrated, but I don't fully understand this. There is no corresponding change in spec.gmk.in adding OPENJDK_TARGET/BUILD_CPU_AUTOCONF and OPENJDK_TARGET/BUILD_ABI. 
> 
> So why the AC_SUBST()?

I thought we need to declare these as `AC_SUBST` as we define the variables outside this file? I can revert these additions if you think these are incorrect.

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PR: https://git.openjdk.java.net/jdk/pull/2833