Undeclared constant 'M_PI' in xvid_vbr macro 'DEG2RAD'

[bug]

Summary of the bug:
Undeclared constant 'M_PI' in xvid_vbr macro 'DEG2RAD'

How to reproduce:
Compiling mencoder in media-autobuild suite (MSYS2/MinGW, GCC 13.2)

xvid_vbr.c: In function 'vbr_init_2pass2':
xvid_vbr.c:44:18: error: 'M_PI' undeclared (first use in this function)
   44 | #define DEG2RAD (M_PI / 180.0)
      |                  ^~~~
xvid_vbr.c:887:53: note: in expansion of macro 'DEG2RAD'
  887 |                                         (1.0 +  sin(DEG2RAD * (state->average_frame * 90.0 / state->alt_curve_low_diff)));
      |                                                     ^~~~~~~
xvid_vbr.c:44:18: note: each undeclared identifier is reported only once for each function it appears in
   44 | #define DEG2RAD (M_PI / 180.0)
      |                  ^~~~
xvid_vbr.c:887:53: note: in expansion of macro 'DEG2RAD'
  887 |                                         (1.0 +  sin(DEG2RAD * (state->average_frame * 90.0 / state->alt_curve_low_diff)));
      |                                                     ^~~~~~~
xvid_vbr.c: In function 'vbr_getquant_2pass2':
xvid_vbr.c:44:18: error: 'M_PI' undeclared (first use in this function)
   44 | #define DEG2RAD (M_PI / 180.0)
      |                  ^~~~
xvid_vbr.c:1250:84: note: in expansion of macro 'DEG2RAD'
 1250 |                                                                                sin(DEG2RAD * ((dbytes - state->average_frame) * 90.0 / state->alt_curve_high_diff)));
      |                                                                                    ^~~~~~~