xz: Prohibited building old lzma parts and fixed build on stage 1. See note!

We stoped touching CFLAGS through makeopts as it caused build failures, configure
not being able to salvage CFLAGS as a precious var. XZ-Utils can only be built with
-std=gnu99 which is defined in its Makefile, but we pass 'CC=gcc' by default and
owerwrite teh needed '-std=gnu99' iso we need to workaround the problem manually.

archiver/xz/xz.conf

@@ -14,4 +14,5 @@
 
 var_append GCC_WRAPPER_APPEND ' ' '-std=gnu99'
 var_append SYSGCC_WRAPPER_APPEND ' ' '-std=gnu99'
-var_append makeopt ' ' 'CFLAGS="-std=gnu99"'
+export CC="$CC -std=gnu99"
+var_append confopt ' ' ' --disable-lzmadec --disable-lzmainfo --disable-lzma-links'