CMake/Find packages: add FindFFmpeg.cmake module.

The behaviour is a bit different from other find modules:
`FFMPEG_FIND_COMPONENTS` should contain the
list of required libraries before calling `find_package(FFmpeg)`.
If not set, a default set of most common libraries is searched for:
`avcodec avdevice avfilter avformat avutil`
http://ffmpeg.org/download.html

Ref D8855

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8918

build_files/cmake/Modules/FindFFmpeg.cmake

@@ -0,0 +1,83 @@
+# - Find FFmpeg library and includes.
+# Set FFMPEG_FIND_COMPONENTS to the canonical names of the libraries
+# before using the package.
+# This module defines
+#  FFMPEG_INCLUDE_DIRS, where to find libavcodec/ac3_parser.h.
+#  FFMPEG_LIBRARIES, libraries to link against to use FFmpeg.
+#  FFMPEG_ROOT_DIR, The base directory to search for FFmpeg.
+#                        This can also be an environment variable.
+#  FFMPEG_FOUND, If false, do not try to use FFmpeg.
+#  FFMPEG_<COMPONENT>_LIBRARY, the given individual component libraries.
+#=============================================================================
+# Copyright 2020 Blender Foundation.
+#
+# Distributed under the OSI-approved BSD 3-Clause License,
+# see accompanying file BSD-3-Clause-license.txt for details.
+#=============================================================================
+
+# If FFMPEG_ROOT_DIR was defined in the environment, use it.
+if(NOT FFMPEG_ROOT_DIR AND NOT $ENV{FFMPEG_ROOT_DIR} STREQUAL "")
+  set(FFMPEG_ROOT_DIR $ENV{FFMPEG_ROOT_DIR})
+endif()
+
+set(_ffmpeg_SEARCH_DIRS
+  ${FFMPEG_ROOT_DIR}
+  /opt/lib/ffmpeg
+)
+
+if(NOT FFMPEG_FIND_COMPONENTS)
+  set(FFMPEG_FIND_COMPONENTS
+    # List taken from http://ffmpeg.org/download.html#build-mac
+    avcodec
+    avdevice
+    avfilter
+    avformat
+    avutil
+  )
+endif()
+
+find_path(_ffmpeg_INCLUDE_DIR
+  NAMES
+    libavcodec/ac3_parser.h
+  HINTS
+    ${_ffmpeg_SEARCH_DIRS}
+  PATH_SUFFIXES
+    include
+)
+
+set(_ffmpeg_LIBRARIES)
+foreach(_component ${FFMPEG_FIND_COMPONENTS})
+  string(TOUPPER ${_component} _upper_COMPONENT)
+  find_library(FFMPEG_${_upper_COMPONENT}_LIBRARY
+    NAMES
+      ${_upper_COMPONENT}
+    HINTS
+      ${LIBDIR}/ffmpeg
+    PATH_SUFFIXES
+      lib64 lib
+  )
+  if(NOT FFMPEG_${_upper_COMPONENT}_LIBRARY)
+    message(WARNING "Could not find FFpeg ${_upper_COMPONENT}.")
+  endif()
+  list(APPEND _ffmpeg_LIBRARIES ${FFMPEG_${_upper_COMPONENT}_LIBRARY})
+  mark_as_advanced(FFMPEG_${_upper_COMPONENT}_LIBRARY)
+endforeach()
+
+# handle the QUIETLY and REQUIRED arguments and set FFMPEG_FOUND to TRUE if
+# all listed variables are TRUE
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(FFmpeg DEFAULT_MSG
+    _ffmpeg_LIBRARIES _ffmpeg_INCLUDE_DIR)
+
+IF(FFMPEG_FOUND)
+  set(FFMPEG_LIBRARIES ${_ffmpeg_LIBRARIES})
+  set(FFMPEG_INCLUDE_DIRS ${_ffmpeg_INCLUDE_DIR})
+ENDIF(FFMPEG_FOUND)
+
+mark_as_advanced(
+  FFMPEG_INCLUDE_DIR
+)
+
+unset(_ffmpeg_SEARCH_DIRS)
+unset(_ffmpeg_LIBRARIES)
+unset(_ffmpeg_INCLUDE_DIR)