fix race condition in meson build

Author: ndgrigorian

meson.build

@@ -101,25 +101,32 @@ if not mkl_dep.found()
     mkl_dep = declare_dependency(dependencies: [mkl_core, mkl_intel_lp64, mkl_tbb_thread])
 endif
 
-# generate loops, similar to numpy
-src_file_cli = find_program('_vendored/process_src_template.py')
-
-src_file = generator(
-    src_file_cli,
-    arguments: ['@INPUT@', '-o', '@OUTPUT@'],
-    output: '@BASENAME@'
-)
-
-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',
     output: '__umath_generated.c',
     input: 'mkl_umath/generate_umath.py',
     command: [py, '@INPUT@', '-o', '@OUTPUT@']
 )
 
+gen_loops_c = custom_target(
+    'gen_loops_c',
+    input: 'mkl_umath/src/mkl_umath_loops.c.src',
+    output: 'mkl_umath_loops.c',
+    command: [
+        py, '_vendored/process_src_template.py', '@INPUT@', '-o', '@OUTPUT@'
+    ]
+)
+
+gen_loops_h = custom_target(
+    'gen_loops_h',
+    input: 'mkl_umath/src/mkl_umath_loops.h.src',
+    output: 'mkl_umath_loops.h',
+    command: [
+        py, '_vendored/process_src_template.py', '@INPUT@', '-o', '@OUTPUT@'
+    ],
+    depends: [gen_umath_c]
+)
+
 mkl_umath_loops = shared_library(
     'mkl_umath_loops',
     sources: [gen_loops_c, gen_loops_h],
@@ -143,7 +150,7 @@ 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, tbb_dep, loops_dep],
     c_args: c_args + ['-DNPY_NO_DEPRECATED_API=NPY_1_7_API_VERSION'],