[Buildroot] liquid-dsp: new package

[Guillaume William Bres]

Liquid-DSP is a DSP library using pure C and only relying on libm.
This patch clones the master branch because the latest tag is quite old,
and configure.ac does not know of armv7|armv8 in that version for instance.

Autoconf requires to include -I(PACKAGE_DIR)/scripts, so I did my best
to have something working, you guys might have suggestions to improve that.

Finally, ./configure does not work with an uclibc toolchain, so I added BR2_TOOLCHAIN_GLIBC
as a dependency. The error is crealf() is not found (<complex.h>). The ARM and NEON
optimizations work fine though.