Add an unoptimised GCC C99 build

This build complains about plain (non-static) inline functions, whose
interpretation differs between gcc -std=gnu90 and C99.  (Ideally we'd
use -std=c99, but that also suppresses declarations for standard but
non-C-standard functions like strdup() and getopt() and always fails.)

Also redo the htslib subproject as a before_script, so $HTSDIR trivia
doesn't appear in the build matrix and we're not overriding the install
script (though that's not being otherwise used anyway).
Use static inline, not just inline

Define these as static inline in the C99-approved way.  Fixes #82.