ArrayFire improvements (#20615)

* ArrayFire: Add version 3.7.2.

* ArrayFire: Allow using MKL as the FFTW provider.

* ArrayFire: Ensure the libraries are properly found.

The required backend(s) can be specified in the library query.

var/spack/repos/builtin/packages/arrayfire/package.py

@@ -16,6 +16,7 @@ class Arrayfire(CMakePackage, CudaPackage):
 
     version('master', submodules=True)
     version('3.7.3', submodules=True, tag='v3.7.3')
+    version('3.7.2', submodules=True, tag='v3.7.2')
     version('3.7.0', submodules=True, tag='v3.7.0')
 
     variant('cuda',   default=False, description='Enable Cuda backend')
@@ -23,7 +24,7 @@ class Arrayfire(CMakePackage, CudaPackage):
     variant('opencl', default=False, description='Enable OpenCL backend')
 
     depends_on('boost@1.65:')
-    depends_on('fftw')
+    depends_on('fftw-api@3:')
     depends_on('blas')
     depends_on('cuda@7.5:', when='+cuda')
     depends_on('cudnn', when='+cuda')
@@ -35,6 +36,22 @@ class Arrayfire(CMakePackage, CudaPackage):
     depends_on('fontconfig', when='+forge')
     depends_on('glfw@3.1.4:', when='+forge')
 
+    @property
+    def libs(self):
+        query_parameters = self.spec.last_query.extra_parameters
+
+        libraries = []
+        if 'cpu' in query_parameters:
+            libraries.append('libafcpu')
+        if 'cuda' in query_parameters and '+cuda' in self.spec:
+            libraries.append('libafcuda')
+        if 'opencl' in query_parameters and '+opencl' in self.spec:
+            libraries.append('libafopencl')
+        if not query_parameters or 'unified' in query_parameters:
+            libraries.append('libaf')
+
+        return find_libraries(libraries, root=self.prefix, recursive=True)
+
     def cmake_args(self):
         args = []
         args.extend([
@@ -45,4 +62,9 @@ def cmake_args(self):
             '-DAF_BUILD_OPENCL={0}'.format(
                 'ON' if '+opencl' in self.spec else 'OFF'),
         ])
+        if '^mkl' in self.spec:
+            args.append('-DUSE_CPU_MKL=ON')
+            if '%intel' not in self.spec:
+                args.append('-DMKL_THREAD_LAYER=GNU OpenMP')
+
         return args