Remove flags that old compilers don't understand.
https://github.com/macports/macports-ports/pull/568
--- doc/educational_decoder/Makefile.orig	2021-12-20 16:49:18.000000000 -0600
+++ doc/educational_decoder/Makefile	2021-12-22 03:35:11.000000000 -0600
@@ -27,7 +27,7 @@
 CFLAGS   += -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow                 \
             -Wstrict-aliasing=1 -Wswitch-enum                               \
             -Wredundant-decls -Wstrict-prototypes -Wundef                   \
-            -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings      \
+            -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings            \
             -std=c99
 CFLAGS   += $(DEBUGFLAGS)
 CFLAGS   += $(MOREFLAGS)
--- lib/libzstd.mk.orig	2021-12-20 16:49:18.000000000 -0600
+++ lib/libzstd.mk	2021-12-22 02:41:39.000000000 -0600
@@ -113,7 +113,7 @@
 DEBUGFLAGS= -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow \
             -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \
             -Wstrict-prototypes -Wundef -Wpointer-arith \
-            -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings \
+            -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings \
             -Wredundant-decls -Wmissing-prototypes -Wc++-compat
 CFLAGS   += $(DEBUGFLAGS) $(MOREFLAGS)
 ASFLAGS  += $(DEBUGFLAGS) $(MOREFLAGS) $(CFLAGS)