Fixes to build

Summary:
Minor fixes to CMake build for flashlight and wav2letter++:
- Use `find_package` instead of `find_dependency` for compatibility
- Only search for flashlight backend-specific dependencies if the backend requiring those dependencies is explicitly selected. This is a stopgap fix; the eventual goal is to always search for all bakcend-specific dependencies and build all flashlight backends for which dependencies exist, but this requires some reconfiguration with teh buidl that will happen down the line.

Reviewed By: vineelpratap

Differential Revision: D13548638

fbshipit-source-id: c3e3224597780bab433a4176cb58c6cd5fe7c28c

Author: jacobkahn

CMakeLists.txt

@@ -81,54 +81,56 @@ else()
 endif()
 
 # -------------------- Locate Backend-specific Dependencies --------------------
-find_package(CUDA 9.2 QUIET) # CUDA 9.2 is required for >= ArrayFire 3.6.1
-if (CUDA_FOUND)
-  message(STATUS "CUDA found (library: ${CUDA_LIBRARIES} include: ${CUDA_INCLUDE_DIRS})")
-else()
-  message(STATUS "CUDA not found")
-  if (FLASHLIGHT_USE_CUDA)
-    message(FATAL_ERROR "CUDA required to build CUDA backend")
-  endif ()
-endif()
-
-find_package(CUDNN 7.2 QUIET) # CUDNN 7.2 works with CUDA 9.2
-if (CUDNN_FOUND)
-  message(STATUS "CUDNN found (library: ${CUDNN_LIBRARIES} include: ${CUDNN_INCLUDE_DIRS})")
-else()
-  message(STATUS "CUDNN not found")
-  if (FLASHLIGHT_USE_CUDA)
-    message(FATAL_ERROR "CUDNN required to build CUDA backend")
-  endif ()
+# TODO: rather than conditionally searching for backend-specific dependencies,
+# always search for all dependencies, and dynamically build all backends for
+# which all required dependencies are found.
+
+if (FLASHLIGHT_USE_CUDA)
+   find_package(CUDA 9.2 QUIET) # CUDA 9.2 is required for >= ArrayFire 3.6.1
+   if (CUDA_FOUND)
+     message(STATUS "CUDA found (library: ${CUDA_LIBRARIES} include: ${CUDA_INCLUDE_DIRS})")
+   else()
+     message(STATUS "CUDA not found")
+     message(FATAL_ERROR "CUDA required to build CUDA backend")
+   endif()
+   
+   find_package(CUDNN 7.2 QUIET) # CUDNN 7.2 works with CUDA 9.2
+   if (CUDNN_FOUND)
+     message(STATUS "CUDNN found (library: ${CUDNN_LIBRARIES} include: ${CUDNN_INCLUDE_DIRS})")
+   else()
+     message(STATUS "CUDNN not found")
+     message(FATAL_ERROR "CUDNN required to build CUDA backend")
+   endif()
 endif()
 
-find_package(MKL QUIET)
-if (MKL_FOUND)
-  message(STATUS "MKL found")
-else()
-  message(STATUS "MKL not found")
-  if (FLASHLIGHT_USE_CPU)
+if (FLASHLIGHT_USE_CPU)
+  find_package(MKL QUIET)
+  if (MKL_FOUND)
+    message(STATUS "MKL found")
+  else()
+    message(STATUS "MKL not found")
     message(FATAL_ERROR "MKL required to build CPU backend")
   endif()
-endif()
 
-find_package(MKLDNN QUIET)
-if (MKLDNN_FOUND)
-  message(STATUS "MKLDNN found")
-else()
-  message(STATUS "MKLDNN not found")
-  if (FLASHLIGHT_USE_CPU)
+  find_package(MKLDNN QUIET)
+  if (MKLDNN_FOUND)
+    message(STATUS "MKLDNN found")
+  else()
+    message(STATUS "MKLDNN not found")
     message(FATAL_ERROR "MKLDNN required to build CPU backend")
   endif()
 endif()
 
-find_package(OpenCL)
-if (OpenCL_FOUND)
-  message(STATUS "OpenCL found (library: ${OpenCL_LIBRARIES} include: ${OpenCL_INCLUDE_DIRS})")
-else()
-  message(STATUS "OpenCL not found")
-  if (FLASHLIGHT_USE_OPENCL)
-    message(FATAL_ERROR "OpenCL required to build OpenCL backend")
-  endif ()
+if (FLASHLIGHT_USE_OPENCL)
+  find_package(OpenCL)
+  if (OpenCL_FOUND)
+    message(STATUS "OpenCL found (library: ${OpenCL_LIBRARIES} include: ${OpenCL_INCLUDE_DIRS})")
+  else()
+    message(STATUS "OpenCL not found")
+    if (FLASHLIGHT_USE_OPENCL)
+      message(FATAL_ERROR "OpenCL required to build OpenCL backend")
+    endif ()
+  endif()
 endif()