configure.ac: Fix checks for cross-compiles.

configure.ac: Fix checks for cross-compiles.

When checking architecture-specific things, always check $host, i.e. the
architecture we're building _for_, not the one we happen to build _on_.

E.g. when cross-compiling _for_ Android (or Windows or others) it's important
to check for Android in $host; whether we happen to cross-compile _on_ a Linux
or Windows or OpenBSD or FreeBSD machine ($build) doesn't matter, only the
fact that we compile _for_ Android is important for most checks.