Prevent:
error: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications  and pre-2001 POSIX applications

diff --git a/configure b/configure
--- a/configure
+++ b/configure
@@ -9759,6 +9759,8 @@
 cross_compiling=$ac_cv_prog_cc_cross
 
     else
+       CFLAGS="$CFLAGS -D_XPG6 -D__EXTENSIONS__"
+       CXXFLAGS="$CXXFLAGS -D_XPG6 -D__EXTENSIONS__"
        LDFLAGS="$LDFLAGS -Wl,-z,ignore -Wl,-R,'\$\$ORIGIN:\$\$ORIGIN/..' -Wl,-z,lazyload -Wl,-z,combreloc -Wl,-z,muldefs"
        LIBS="-lc $LIBS"
        MKSHLIB_FORCE_ALL='-Wl,-z -Wl,allextract'