gen.py: Pass -pthread, not -lpthread, in ldflags.

This was shamefully overlooked in 1152229e ("gen.py: Pass -pthread via
ldflags, not libs"), where I was supposed to revert the entirety of commit
082fbe39 ("Pass -lpthread when linking instead of -pthread") but did not
notice that 0ac93dd2 ("For proper order put -lpthreads as a 'library' not a
'linker flag'") continued passing -lpthread instead of -pthread.

Bug: chromium:934793
Change-Id: I1fa6d2c8b32a86d621a74ae97f391fbe4ea5e989
Reviewed-on: https://gn-review.googlesource.com/c/4100
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
diff --git a/build/gen.py b/build/gen.py
index c426c0b..a7142fa 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -334,7 +334,7 @@
       ldflags.append('-maix64')
 
     if platform.is_posix():
-      ldflags.append('-lpthread')
+      ldflags.append('-pthread')
 
     if options.use_lto:
       cflags.extend(['-flto', '-fwhole-program-vtables'])