Check compiler support for -fno-default-inline and -pipe

clang was spewing warnings about the unrecognized -fno-default-inline.  (Oddly,
it warns only with -c, not when compiling directly to an executable.)  For
completeness we also check -pipe, even though clang is OK with that one.

It should be fine to omit either flag.  gcc -fno-default-inline drops the
implicit 'inline' annotation on functions defined inside a class scope, but
'inline' is only a hint anyway.  -fno-default-inline does not change linkage.
-pipe is merely a compile speed optimization.