Add builder for Opus

[giordano]

Dependency of sdl2_mixer (#122).

This doesn't work only on aarch64, during make I get:

libtool: compile:  cc -std=gnu99 -DHAVE_CONFIG_H -I. -I./include -I./celt -I./silk -I./silk/float -I./silk/fixed -g -O2 -fvisibility=hidden -D_FORTIFY_SOURCE=2 -W -Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes -MT celt/arm/celt_neon_intr.lo -MD -MP -MF celt/arm/.deps/celt_neon_intr.Tpo -c celt/arm/celt_neon_intr.c  -fPIC -DPIC -o celt/arm/.libs/celt_neon_intr.o
In file included from celt/arm/celt_neon_intr.c:37:0:
celt/arm/celt_neon_intr.c: In function ‘xcorr_kernel_neon_float’:
celt/arm/celt_neon_intr.c:137:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YY[0], vget_low_f32(XX[0]), 0);
              ^
celt/arm/celt_neon_intr.c:139:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[0], vget_low_f32(XX[0]), 1);
              ^
celt/arm/celt_neon_intr.c:141:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[1], vget_high_f32(XX[0]), 0);
              ^
celt/arm/celt_neon_intr.c:143:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[2], vget_high_f32(XX[0]), 1);
              ^
celt/arm/celt_neon_intr.c:145:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YY[1], vget_low_f32(XX[1]), 0);
              ^
celt/arm/celt_neon_intr.c:147:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[0], vget_low_f32(XX[1]), 1);
              ^
celt/arm/celt_neon_intr.c:149:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[1], vget_high_f32(XX[1]), 0);
              ^
celt/arm/celt_neon_intr.c:151:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[2], vget_high_f32(XX[1]), 1);
              ^
celt/arm/celt_neon_intr.c:170:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YY[0], vget_low_f32(XX[0]), 0);
              ^
celt/arm/celt_neon_intr.c:172:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[0], vget_low_f32(XX[0]), 1);
              ^
celt/arm/celt_neon_intr.c:174:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[1], vget_high_f32(XX[0]), 0);
              ^
celt/arm/celt_neon_intr.c:176:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YEXT[2], vget_high_f32(XX[0]), 1);
              ^
celt/arm/celt_neon_intr.c:184:14: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
       SUMM = vmlaq_lane_f32(SUMM, YY[0], XX_2, 0);
              ^
celt/arm/celt_neon_intr.c:189:11: error: incompatible types when initializing type ‘float32x4_t’ using type ‘float32x2_t’
    SUMM = vmlaq_lane_f32(SUMM, YY[0], XX_2, 0);
           ^