glfw-devel: First try to force a C11 compliant compiler to be used

GLFW requires -std=C11 support as of 8055dad (20190305 -- NOTE this is the effective date, not the commit date), for <stdatomic.h>.

graphics/glfw/Portfile

@@ -93,6 +93,22 @@ if {${os.platform} eq "darwin" && ${os.major} == 10} {
 
         patchfiles-append patch-src_CMakeLists.txt.devel.diff
 
+        # requires c11 support as of 82ca58da (20190305) for
+        # <stdatomic.h>.  hopefully the following force a C11
+        # compliant compiler to be used!  require GCC >= 4.6 (4.9 for
+        # full C11) or MacPorts' Clang >= 3.1 or AppleClang >= 700.
+        # MacPorts' Clang starts at 3.3, so nothing to block there!
+
+        PortGroup compiler_blacklist_versions 1.0
+
+        compiler.blacklist-append macports-gcc-4.3 macports-gcc-4.4 macports-gcc-4.5 \
+            macports-gcc macports-gcc-4.6 macports-gcc-4.7 macports-gcc-4.8 \
+            macports-llvm-gcc-4.2 macports-dragonegg-3.3 macports-dragonegg-3.4 \
+            apple-gcc-4.0 apple-gcc-4.2 gcc-3.3 gcc-4.0 llvm-gcc-4.2 gcc cc \
+            {clang < 700}
+
+        configure.args-append -DCMAKE_C_STANDARD=11 -DCMAKE_C_EXTENSIONS=OFF
+
     }
 }