lavfi/af_aemphasis: remove unnecessary complex number usage

complex is not available on all platforms. Furthermore, it is trivial to
rewrite complex number expressions to real arithmetic, and in fact
sometimes advantageous for performance reasons: by wrapping as a complex,
one forces a particular Cartesian representation that is not necessarily optimal for the purpose.

Configure dependencies also removed, and aemphasis is now available across
all platforms.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

configure

@@ -2836,7 +2836,6 @@ unix_protocol_deps="sys_un_h"
 unix_protocol_select="network"
 
 # filters
-aemphasis_filter_deps="cabs cexp"
 amovie_filter_deps="avcodec avformat"
 aresample_filter_deps="swresample"
 ass_filter_deps="libass"

libavfilter/af_aemphasis.c

@@ -18,8 +18,6 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include <complex.h>
-
 #include "libavutil/opt.h"
 #include "avfilter.h"
 #include "internal.h"
@@ -189,14 +187,15 @@ static inline void set_lp_rbj(BiquadD2 *bq, double fc, double q, double sr, doub
 
 static double freq_gain(BiquadCoeffs *c, double freq, double sr)
 {
-    double complex z, w;
+    double zr, zi;
 
     freq *= 2.0 * M_PI / sr;
-    w = 0 + I * freq;
-    z = 1.0 / cexp(w);
+    zr = cos(freq);
+    zi = -sin(freq);
 
-    return cabs(((double complex)c->a0 + c->a1 * z + c->a2 * z*z) /
-                ((double complex)1.0 + c->b1 * z + c->b2 * z*z));
+    /* |(a0 + a1*z + a2*z^2)/(1 + b1*z + b2*z^2)| */
+    return hypot(c->a0 + c->a1*zr + c->a2*(zr*zr-zi*zi), c->a1*zi + 2*c->a2*zr*zi) /
+           hypot(1 + c->b1*zr + c->b2*(zr*zr-zi*zi), c->b1*zi + 2*c->b2*zr*zi);
 }
 
 static int config_input(AVFilterLink *inlink)