When using meson to build glib git master, pthread_setname_np prototype is misdetected and build fails with the following (ignore the version, it is git master from today):

clang  -Iglib/glib-2.0@sha -Iglib -I../glib-2.53.4/glib -I. -I../glib-2.53.4/ -Xclang -fcolor-diagnostics -pipe -D_FILE_OFFSET_BITS=64 -std=gnu89 -march=skylake -fomit-frame-pointer -fstack-protector-strong -O2 -fPIC -pthread '-DG_LOG_DOMAIN="GLib"' -DGLIB_COMPILATION -DPCRE_STATIC -fvisibility=hidden -MMD -MQ 'glib/glib-2.0@sha/gthread-posix.c.o' -MF 'glib/glib-2.0@sha/gthread-posix.c.o.d' -o 'glib/glib-2.0@sha/gthread-posix.c.o' -c ../glib-2.53.4/glib/gthread-posix.c
../glib-2.53.4/glib/gthread-posix.c:1231:27: error: too few arguments to function call, expected 2, have 1
  pthread_setname_np (name); /* on OS X and iOS */
  ~~~~~~~~~~~~~~~~~~      ^
/usr/include/pthread.h:452:1: note: 'pthread_setname_np' declared here
extern int pthread_setname_np (pthread_t __target_thread, const char *__name)


Inside meson-log.txt, there's the following:

Running compile:
Working directory:  /tmp/tmppgses23b
Command line:  clang /tmp/tmppgses23b/testfile.c -march=skylake -fomit-frame-pointer -fstack-protector-strong -O2 -pipe -O0 -std=gnu89 -D_FILE_OFFSET_BITS=64 -o /tmp/tmppgses23b/output.exe 

Code:
 #include <pthread.h>
                 int main() {
                   pthread_setname_np("example");
                 }
Compiler stdout:
 
Compiler stderr:
 
Checking if "pthread_setname_np(const char*)" links: YES


Now, if I manually compile that program with clang, I get the following

$ cat test.c
 #include <pthread.h>
                 int main() {
                   pthread_setname_np("example");
                 }

$ gcc test.c -pthread
test.c: In function ‘main’:
test.c:3:20: warning: implicit declaration of function ‘pthread_setname_np’; did you mean ‘pthread_setcanceltype’? [-Wimplicit-function-declaration]
                    pthread_setname_np("example");
                    ^~~~~~~~~~~~~~~~~~
                    pthread_setcanceltype
$ gcc -std=c89 test.c -pthread

$ clang test.c -pthread

test.c:3:20: warning: implicit declaration of function 'pthread_setname_np' is invalid in C99 [-Wimplicit-function-declaration]
                   pthread_setname_np("example");
                   ^
1 warning generated.

$ clang -std=c89 test.c -pthread

As you can see, in c89 mode (which meson forces), no warning or error will be generated with either clang or gcc.

This is gcc 7.1.1 20170727, clang 4.0.1, glibc 2.26, binutils 2.29

PKGBUILD and meson flags can be found at

https://github.com/elkrejzi/pacman/blob/master/pkgbuild/glib/PKGBUILD.meson