It only existed to not include -lm in the .pc for fixed point builds,
but that is still needed since the float API is still enabled and
will use at least lrint.
Doing it this way means it's only the weird corner case that actually
pays for being weird, and we don't litter the build dir with an extra
link that normal builds really don't need at all.
It also avoids the problem of platforms where LN_S isn't well defined.
The floating-point build calls a number of math library
functions, and linking with libm is technically necessary.
It wasn't obvious because most systems support shlib
dependencies and pulled it in that way, or supply the
referenced functions with intrinsics. Discovered the issue
trying to build libopusfile against the uninstalled static
libopus, which unlike opus-tools, doesn't itself need libm.
The -lm argument is only added to Libs for the floating-
point (default) build. It's not necessary for the fixed-
point build.
Also mark which build was used in the .pc file description.
It's less surprising for people using the -uninstalled
pkg-config file if we advertise the static library rather
than the shared build. That way the executables work without
having to hack the same location into LD_LIBRARY_PATH, and
the resulting binaries are easier to distribute.
For this to actually work with opus-tools, the -fPIE had
to be turned off, as it was in 1d128b4d5c6f. Support
for static PIE builds would be restored if we compiled
libopus.a with -fPIC.
Also remove the private header locations. These are no
longer required to declare any of the public api.
These files allow compiling and linking against the reference
implementation with an invocation like `pkg-config --cflags
--libs opus`. They're optional within the opus build system,
but can simplify compilation for clients and are generally
considered part of a normal autotools build these days.
The -uninstalled version refers to local build products
within the tree, which is helpful for testing and unprivileged
builds. opus.pc is used once 'make install' has put the
libraries and headers somewhere more permanent.
