Replacing c2x by c1x in some files

[uNouss]

In these files

• syntax/trunk/etc/bin/sxmake

  Line 538 in 2e0a6fd

  GCC_WARNINGS="-Wall -Wextra -Wswitch-default -Wdeclaration-after-statement -Wundef -Wshadow -Wbad-function-cast -Wcast-qual -Wredundant-decls -Wno-char-subscripts -Wcast-align -fno-common -Wstrict-prototypes -Wformat -Wuninitialized -fstrict-aliasing -Wold-style-definition -std=c2x"

• syntax/trunk/etc/make/gccoptions

  Line 21 in 2e0a6fd

  GCC_WARNINGS=-Wall -Wextra -Wswitch-default -Wdeclaration-after-statement -Wundef -Wshadow -Wbad-function-cast -Wcast-qual -Wredundant-decls -Wno-char-subscripts -Wcast-align -fno-common -Wstrict-prototypes -Wformat -Wuninitialized -fstrict-aliasing -Wold-style-definition -std=c2x

we have -std=c2x, and this option is not compatible with most versions of gcc tested, unlike -std=c1x.

Instead of just replacing it, it might be a better option to declare the GCC_WARNINGS variable once.