Ignore -flto=* CFLAGS

[yutannihilation]

gcc-ASAN build is failing. It seems this is because gcc-ASAN build adds -flto=10; since the latest Makevars.in passes CFLAGS to the cc crate, it blows away the symbol. We should not let it get optimized at this point.

** testing if installed package can be loaded from temporary location
Error: package or namespace load failed for ‘string2path’ in dyn.load(file, DLLpath = DLLpath, ...):
 unable to load shared object '/data/gannet/ripley/R/packages/tests-gcc-SAN/string2path.Rcheck/00LOCK-string2path/00new/string2path/libs/string2path.so':
  /data/gannet/ripley/R/packages/tests-gcc-SAN/string2path.Rcheck/00LOCK-string2path/00new/string2path/libs/string2path.so: undefined symbol: unwind_protect_impl
Error: loading failed
Execution halted
ERROR: loading failed