RFR: 8256012: Fix build of Monocle for Linux

[John Neffenger]

On Tue, 24 Nov 2020 21:18:03 GMT, Johan Vos <jvos at openjdk.org> wrote:

> I don't see any harm in this PR, but I wonder which toolchains are still not having `RTLD_NEXT` without specifying GNU_SOURCE.

Thanks, Johan. I've been following the instructions in the [OpenJFX Wiki][1]. I use the GCC cross-compiler installed by the repository's [crosslibs-armv6hf.sh][2] script. Then I build for ARM using the repository's [armv6hf.gradle][3] build file.

> Does it also work if you specify `-D__USE_GNU` to the compiler?

No, that prints the same error. The comments in the header file `/usr/include/features.h` suggest that the macro `__USE_GNU` is for internal use, while `_GNU_SOURCE` is defined by the user. I tried the user macro, and it works!

So below is an alternative fix in the ARM build file:

diff --git a/buildSrc/armv6hf.gradle b/buildSrc/armv6hf.gradle
index 05e3e83551..bd56dcf86c 100644
--- a/buildSrc/armv6hf.gradle
+++ b/buildSrc/armv6hf.gradle
@@ -140,7 +140,7 @@ def iioCFlags = [extraCFlags,
          ].flatten()
 def iioLFlags = [extraLFlags].flatten()

-def es2EglfbCFlags = [extraCFlags, "-DIS_EGLFB", "-DLINUX"].flatten()
+def es2EglfbCFlags = [extraCFlags, "-DIS_EGLFB", "-DLINUX", "-D_GNU_SOURCE"].flatten()
 def es2EglfbLFlags = [extraLFlags].flatten()
 def es2MonocleCFlags = [extraCFlags, "-DIS_EGLFB", "-DLINUX"].flatten()
 def es2MonocleLFlags = [extraLFlags].flatten()

It seems the [`_GNU_SOURCE` macro does a lot][4], so it's a question of limiting its use to one C file or enabling it for the entire `prism_es2_monocle` library. I'm fine with either solution.

[1]: https://wiki.openjdk.java.net/display/OpenJFX/Cross+Building+for+ARM+Hard+Float
[2]: https://github.com/openjdk/jfx/blob/master/buildSrc/crosslibs/crosslibs-armv6hf.sh
[3]: https://github.com/openjdk/jfx/blob/master/buildSrc/armv6hf.gradle
[4]: https://stackoverflow.com/q/5582211

-------------

PR: https://git.openjdk.java.net/jfx/pull/350