While I think that building with GCC 4 on Solaris 11/12 is minor issue, requirement of newer POSIX API (on GNU/Linux) would be a bit serious issue.
I pushed my change to fix this.

While original npth-1.6 can be compiled with newer gcc (>= 5), we'd say please use CFLAGS+=-std=gnu99 with older gcc, as workaround.

I figured out:

• Removing -D_POSIX_C_SOURCE=200112L works both of gcc 4.9 and gcc 5.5 on Solaris 11.3 (even with -std=c99).
• Then, adding -D_XOPEN_SOURCE=500, gcc 4.9 works, but gcc 5.5 failed by another error (Compiler or options invalid for pre-UNIX 03 X/Open applications and pre-2001 POSIX applications)
  • I confirmed gcc 5.5 defaults to -std=gnu99

Thanks for your offer. I have an account for GCC Compiler Farm. I'm trying with gcc211 machine. will back soon.

In case of gcc 4.8 on Solaris, could you please try this patch (instead of configure patch) to see if it works?

It looks like somewhat complicated more. It seems that specifying _POSIX_C_SOURCE=200112L is not good on Solaris with old GCC. Perhaps, it would have no problem with newer gcc (or -std=gnu99 option).

IIUC, -std=c99 won't solve this issue. It is Solaris specific C99 issue.

It looks like Solaris only needs CFLAGS+=-std=c99. It was added for all programs and libraries listed at https://www.gnupg.org/download/index.html.

OK, it's a libc with no pthread_rwlock_t.

New gpg-error.m4 detects gpgrt-config, too.
And configure supplies --libdir when it invokes gpgrt-config.
For other *.m4 (libassuan, ksba, libgcrypt, ntbtls), it is possible for them to check GPGRT_CONFIG to use gpgrt-config if any.
For npth.m4, it can do that too, with no hard dependency to libgpg-error.

A bit tricky, but this would be good to use gpgrt-config by gpg-error.m4.
I say "tricky", because its name is gpg-error.m4 but it configure GPGRT_CONFIG to access to GPG_ERROR_CONFIG.

It might be good idea to provide libgcrypt.pc in libgcrypt 1.8.x for forward compatibility with libgpg-error 1.33.

Well, I changed my mind. Use of new gpgrt-config requires software update to introduce gpgrt.m4 and update of configure.ac to switch gpgrt from gpg-error, in standard way.
That's too much this time. It's good to defer this change.

OK, I'll change to use gpgrt-config, along with requiring newer version of libgpg-error.

May I suggest to use a (new) gpgrt-config instead of the current name libgpg-error-config. The long term plan is to change the name of the library.

This is the dependency graph:

Thanks, that allowed npth to make successfully without the unsatisfied symbols.

Apparently, the check of sem_init function was not done (in config.log).
Could you please make sure to update npth/configure by npth/autogen.sh?

config.log is attached.

Could you please put the config.log of npth with the patch?
The intention of change is: we need to link -lpthread and -lrt

I tried with the updated patch, but I still see the same unsatisfied symbols during link. I verified that the patch was in place in configure.ac and also patched a clean version of configure.ac so that there would be only one instance of hpux in the case statement:

I realized that the test case is already there.
I'm not sure the reason why make check for npth works well on HP-UX (before the my patch). It uses npth_attr_init (hence, pthread_attr_init) in tests/t-thread.c.
Perhaps, libtool is clever enough to detect -lpthread into src/libnpth.la (dependency_libs), I suppose.

Thanks for your testing, it's near. Here is updated patch:

Rebuilding npth results in three unsatisfied symbols:

Thanks for config.log of GnuPG. I think that I located the problem; While gpg-agent should be linked to -lpthread, it was not. The configure variable NPTH_LIBS in config.log doesn't have -lpthread. Thus, pthread_* are linked to the ones of stub, and it resulted the error.