CDRIVER-1104 Don't error when adding -pthread on clang

Author: bjori

build/autotools/PlatformFlags.m4

@@ -6,6 +6,8 @@ AS_IF([test "$os_darwin" = "yes"],
       [AX_CHECK_COMPILE_FLAG([-Wno-cast-align], [CFLAGS="$CFLAGS -Wno-cast-align"])])
 AS_IF([test "$os_darwin" = "yes"],
       [AX_CHECK_COMPILE_FLAG([-Wno-unneeded-internal-declaration], [CFLAGS="$CFLAGS -Wno-unneeded-internal-declaration"])])
+AS_IF([test "$os_darwin" = "yes"],
+      [AX_CHECK_COMPILE_FLAG([-Wno-error=unused-command-line-argument], [CFLAGS="$CFLAGS -Wno-error=unused-command-line-argument"])])
 dnl We know there are some cast-align issues on Solaris
 AS_IF([test "$os_solaris" = "yes"],
       [AX_CHECK_COMPILE_FLAG([-Wno-cast-align], [CFLAGS="$CFLAGS -Wno-cast-align"])])