Fixing the openmp interface calls to avoid build failures when openmp…

… is not enabled.

viprs/model/vi/e_step.pyx

@@ -1,5 +1,4 @@
 from cython.parallel import prange, parallel
-cimport openmp
 from ...utils.math_utils cimport (
     sigmoid,
     softmax,
@@ -16,6 +15,24 @@ cimport numpy as np
 from cython cimport floating, integral
 
 
+# A safe way to get the number of the thread currently executing the code:
+# This is used to avoid compile-time errors when compiling the code with OpenMP support disabled.
+# In earlier iterations, we used:
+# cimport openmp
+# openmp.omp_get_thread_num()
+# But this tends to fail when OpenMP is not enabled.
+# The code below is a safer way to get the thread number.
+cdef extern from *:
+    """
+    #ifdef _OPENMP
+    #include <omp.h>
+    #else
+    int omp_get_thread_num() { return 0; }
+    #endif
+    """
+    int omp_get_thread_num() noexcept nogil
+
+
 @cython.boundscheck(False)
 @cython.wraparound(False)
 @cython.nonecheck(False)
@@ -243,7 +260,7 @@ cpdef void e_step_mixture(int[::1] ld_left_bound,
     for j in prange(c_size, nogil=True, schedule='static', num_threads=threads):
 
         # Set the thread offset for the u_j array:
-        thread_offset = openmp.omp_get_thread_num() * (K + 1)
+        thread_offset = omp_get_thread_num() * (K + 1)
 
         # The start and end coordinates for the flattened LD matrix:
         ld_start = ld_indptr[j]