package/liquid-dsp: fix build without neon

Fix the following build failure without neon raised since bump to
version 1.4.0 in commit 2f7f8f381394207bfb029ec395f5ced5695a0a2e and
https://github.com/jgaeddert/liquid-dsp/commit/c821187dd9af2758c8e3c82e95eaf041b8ac81a0:

/home/peko/autobuild/instance-0/output-1/host/bin/arm-none-linux-gnueabi-gcc -std=gnu11 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2 -g0 -D_FORTIFY_SOURCE=2  -ffast-math -mcpu=cortex-a7 -mfloat-abi=hard -mfpu=neon-vfpv4  -Wall -fPIC -Wno-deprecated -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I. -Iinclude  -c -o src/audio/src/cvsd.o src/audio/src/cvsd.c
In file included from /home/peko/autobuild/instance-0/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/features.h:388:0,
                 from /home/peko/autobuild/instance-0/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/stdlib.h:24,
                 from src/libliquid.c:25:
/home/peko/autobuild/instance-0/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/gnu/stubs.h:10:29: fatal error: gnu/stubs-hard.h: No such file or directory
 # include <gnu/stubs-hard.h>
                             ^

Indeed, upstream considers that NEON is available on all ARM platforms,
and their configure.ac contains that code snippet:
  239     arm|armv7*|armv8*)
  240         # assume neon instructions are available
  241         # TODO: check for Neon availability
  242
  243         # ARM architecture : use neon extensions

Fixes:
 - http://autobuild.buildroot.org/results/36b3c2220c462e7a20262fd1b9064d9aeb6c9ec4
 - http://autobuild.buildroot.org/results/881826b4b6c141e59a0da2d7d1ad55d3709fdb95

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - refactor with LIQUID_DSP_SIMDOVERRIDE
  - add comment about --disable-simdoverride
  - extend commit log with upstream code snippet
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/liquid-dsp/liquid-dsp.mk

@@ -37,7 +37,18 @@ endif
 
 # disable altivec, it has build issues
 ifeq ($(BR2_powerpc)$(BR2_powerpc64)$(BR2_powerpc64le),y)
+LIQUID_DSP_SIMDOVERRIDE = y
+endif
+
+# Upstream expects NEON on all ARM CPUs by default
+ifeq ($(BR2_arm):$(BR2_ARM_FPU_NEON),y:)
+LIQUID_DSP_SIMDOVERRIDE = y
+endif
+
+ifeq ($(LIQUID_DSP_SIMDOVERRIDE),y)
 LIQUID_DSP_CONF_OPTS += --enable-simdoverride
+# Do not add an else-clause to pass --disable-simdoverride, as it is
+# non-functional, and behaves as --enable-simdoverride
 endif
 
 LIQUID_DSP_CONF_OPTS += \