PR41601 devel/glib2 fails to compile on Solaris 10

[Sad Clouds <cryintothebluesky%gmail.com@localhost>]

I'm hitting the same bug on Solaris 11.3 and pkgsrc-2019Q3

I wonder if the bug needs to be reopened. Does anyone know what causes
this failure? Is this related to -std=gnu89 or some other macro defined
somewhere?

[1/573] Compiling C object 'glib/libcharset/6e4c96c@@charset@sta/localcharset.c.o'.
FAILED: glib/libcharset/6e4c96c@@charset@sta/localcharset.c.o 
gcc -Iglib/libcharset/6e4c96c@@charset@sta -Iglib/libcharset -I../glib/libcharset -I. -I../ -I/opt/pkg/include -I/usr/include -I/opt/pkg/include/python3.7 -fdiagnostics-color=always -pipe -std=gnu89 -D_GNU_SOURCE -fno-strict-aliasing -Wall -Wduplicated-branches -Wimplicit-fallthrough -Wmisleading-indentation -Wstrict-prototypes -Wunused -Wno-bad-function-cast -Wno-cast-function-type -Wno-pedantic -Werror=declaration-after-statement -Werror=format=2 -Werror=implicit-function-declaration -Werror=init-self -Werror=missing-include-dirs -Werror=missing-prototypes -Werror=pointer-arith -O2 -D_FORTIFY_SOURCE=2 -fPIC '-DGLIB_CHARSETALIAS_DIR="lib"' -fvisibility=hidden -MD -MQ 'glib/libcharset/6e4c96c@@charset@sta/localcharset.c.o' -MF 'glib/libcharset/6e4c96c@@charset@sta/localcharset.c.o.d' -o 'glib/libcharset/6e4c96c@@charset@sta/localcharset.c.o' -c ../glib/libcharset/localcharset.c
In file included from /opt/gcc-9.2.0-sparc64/lib/gcc/sparc64-sun-solaris2.11/9.2.0/include-fixed/stdio.h:24,
                 from ../glib/libcharset/localcharset.c:28:
/opt/gcc-9.2.0-sparc64/lib/gcc/sparc64-sun-solaris2.11/9.2.0/include-fixed/sys/feature_tests.h:363:2: error: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications     and pre-2001 POSIX applications"
  363 | #error "Compiler or options invalid for pre-UNIX 03 X/Open applications \
      |  ^~~~~
ninja: build stopped: subcommand failed.
*** Error code 1

Stop.
bmake[4]: stopped in /opt/pkgbuild/pkgsrc/devel/glib2
*** Error code 1