Revert "main/gcc6: delete in favor of Alpine's gcc6"

This reverts commit d56a2facfb
and changes the main/gcc6 pkgrel to 9999, so it will always be higher
than Alpine's version.

main/gcc6/003_all_default-fortify-source.patch

@@ -0,0 +1,40 @@
+# DP: Turn on -D_FORTIFY_SOURCE=2 by default for C, C++, ObjC, ObjC++,
+# DP: if the optimization level is > 0
+
+---
+ gcc/doc/invoke.texi          |    6 ++++++
+ gcc/c-family/c-cppbuiltin.c  |    3 +
+ 2 files changed, 9 insertions(+), 0 deletions(-)
+
+Index: b/gcc/doc/invoke.texi
+===================================================================
+--- a/gcc/doc/invoke.texi
++++ b/gcc/doc/invoke.texi
+@@ -7840,6 +7840,12 @@ also turns on the following optimization
+ Please note the warning under @option{-fgcse} about
+ invoking @option{-O2} on programs that use computed gotos.
+ 
++NOTE: In Alpine Linux, @option{-D_FORTIFY_SOURCE=2} is
++set by default, and is activated when @option{-O} is set to 2 or higher.
++This enables additional compile-time and run-time checks for several libc
++functions.  To disable, specify either @option{-U_FORTIFY_SOURCE} or
++@option{-D_FORTIFY_SOURCE=0}.
++
+ @item -O3
+ @opindex O3
+ Optimize yet more.  @option{-O3} turns on all optimizations specified
+Index: b/gcc/c-family/c-cppbuiltin.c
+===================================================================
+--- a/gcc/c-family/c-cppbuiltin.c
++++ b/gcc/c-family/c-cppbuiltin.c
+@@ -1176,6 +1176,10 @@ c_cpp_builtins (cpp_reader *pfile)
+   builtin_define_with_value ("__REGISTER_PREFIX__", REGISTER_PREFIX, 0);
+   builtin_define_with_value ("__USER_LABEL_PREFIX__", user_label_prefix, 0);
+ 
++  /* Fortify Source enabled by default for optimization levels > 0 */
++  if (optimize)
++    builtin_define_with_int_value ("_FORTIFY_SOURCE", 2);
++
+   /* Misc.  */
+   if (flag_gnu89_inline)
+     cpp_define (pfile, "__GNUC_GNU_INLINE__");