The strdup() function is available on uClibc if _XOPEN_SOURCE_EXTENDED
is defined; since we're using _GNU_SOURCE elsewhere to enable extended
libc features, and uClibc will set _XOPEN_SOURCE_EXTENDED if _GNU_SOURCE
is set, let's use that.

Closes #181

Our caller may load (eg) epoxy_glAlphaFunc, which is a function pointer,
and then call through that value multiple times. Until the caller
re-examines the value of that function pointer, which is a copy
relocation in the executable, repeated calls mean repeated work
resolving the GL function.

We can't make the caller reinspect the variable, but the resolver
function can avoid doing redundant work.

Fixes: anholt/libepoxy#171
Signed-off-by: Adam Jackson <ajax@redhat.com>

Epoxy updates the function pointers in order to avoid calling the
resolver multiple times, but with -Bsymbolic we're going to update the
copy inside libepoxy, instead of the relocated copy in the code using
libepoxy. This leads to libepoxy constantly querying the function
resolver code instead of just once.

We still want to avoid intra-library relocations for our functions,
but we need to live with them for our global function pointers.

See issue #171

We want to consistently handle exceptions for the internal state checks;
calling `exit()` does not allow us to attach a debugger and get a proper
trace.

Fix dlwrap for FreeBSD

Make epoxy_load_{egl,glx} load libraries if they exist

This fixes dlwrap_real_dlsym on FreeBSD, but tests still fail because we
currently don't have libglvnd.

FreeBSD hides declarations of non-POSIX functions when POSIX macros,
such as _POSIX_C_SOURCE or _XOPEN_SOURCE, is defined. This causes
test/dlwrap.c to fail to compile because it uses dlvsym and asprintf.

Fixes: #169

Now that we're being conservative about probing libraries, these
entrypoints would not succeed unless the caller had already dlopened
stuff themselves, or had explicitly linked against the provider library.
Both of those are exactly not what we want.

Signed-off-by: Adam Jackson <ajax@redhat.com>