font_browser.cxx uses pthread_create and pthread_t, but never includes <pthread.h>
The header is likely pulled from a different header that includes it; however, on stricter/newer compilers it will fail (It needs the header to be explicity included in the file it is called in)

--- a/src/widgets/font_browser.cxx
+++ b/src/widgets/font_browser.cxx
@@ -33,6 +33,7 @@
 #include <string>
 #include <cstdio>
 #include <stdint.h>
+#include <pthread.h>

 #include <FL/Fl.H>
 #include <FL/Fl_Color_Chooser.H>