fix race condition in meson build

Author: ndgrigorian

meson.build

@@ -96,7 +96,6 @@ src_file = generator(
 )
 
 gen_loops_c = src_file.process('mkl_umath/src/mkl_umath_loops.c.src')
-gen_loops_h = src_file.process('mkl_umath/src/mkl_umath_loops.h.src')
 
 gen_umath_c = custom_target(
     '__umath_generated',
@@ -105,6 +104,14 @@ gen_umath_c = custom_target(
     command: [py, '@INPUT@', '-o', '@OUTPUT@']
 )
 
+gen_loops_h = custom_target(
+    'mkl_umath_loops_h',
+    output: ['mkl_umath_loops.h'],
+    input: 'mkl_umath/src/mkl_umath_loops.h.src',
+    command: [src_file_cli, '@INPUT@', '-o', '@OUTPUT@'],
+    depends: gen_umath_c
+)
+
 mkl_umath_loops = shared_library(
     'mkl_umath_loops',
     sources: [gen_loops_c, gen_loops_h],
@@ -128,7 +135,7 @@ endif
 # gen_loops_h is generated by custom target so must be included in sources
 py.extension_module(
     '_ufuncs',
-    sources: ['mkl_umath/src/ufuncsmodule.c', gen_umath_c, gen_loops_h],
+    sources: ['mkl_umath/src/ufuncsmodule.c', gen_loops_h],
     include_directories: inc_np,
     dependencies: [mkl_dep, loops_dep],
     c_args: c_args + ['-DNPY_NO_DEPRECATED_API=NPY_1_7_API_VERSION'],