Allow to pass custom `CFLAGS` options via environment variable
`LIBSSH2_CFLAG_EXTRAS`. Default and automatically added options of
`GNUmakefile` have preference over custom ones. This addition is useful
for passing f.e. custom CPU tuning or LTO optimization (`-flto
-ffat-lto-objects`) options. The only current way to do this is to edit
`GNUmakefile`. This patch makes it unnecessary.
This is a mirror of similar libcurl patch:
https://github.com/bagder/curl/pull/136
8f00a7471d