build: search for libbz2 only if FFmpeg or FreeType is enabled. (#4505)

Detect libbz2 only if necessary, i.e., FFmpeg or FreeType is enabled.
This avoids an unnecessary dependency on libbz2 if these features are
disabled.

In particular, with vcpkg, OpenImageIO might pick up a dependency on
libbz2.so from the host system since bzip2 from vcpkg is not listed as a
dependency of OpenImageIO (if the FFmpeg and FreeImage features are
disabled, and bzip2 is not already installed for other reasons).

Signed-off-by: Joachim Reichel <43646584+jreichel-nvidia@users.noreply.github.com>

Author: jreichel-nvidia

src/cmake/externalpackages.cmake

@@ -121,11 +121,6 @@ endif ()
 
 checked_find_package (PNG VERSION_MIN 1.6.0)
 
-checked_find_package (BZip2)   # Used by ffmpeg and freetype
-if (NOT BZIP2_FOUND)
-    set (BZIP2_LIBRARIES "")  # TODO: why does it break without this?
-endif ()
-
 checked_find_package (Freetype
                       VERSION_MIN 2.10.0
                       DEFINITIONS USE_FREETYPE=1 )
@@ -191,6 +186,13 @@ checked_find_package (R3DSDK NO_RECORD_NOTFOUND)  # RED camera
 set (NUKE_VERSION "7.0" CACHE STRING "Nuke version to target")
 checked_find_package (Nuke NO_RECORD_NOTFOUND)
 
+if (FFmpeg_FOUND OR FREETYPE_FOUND)
+    checked_find_package (BZip2)   # Used by ffmpeg and freetype
+    if (NOT BZIP2_FOUND)
+        set (BZIP2_LIBRARIES "")  # TODO: why does it break without this?
+    endif ()
+endif()
+
 
 # Qt -- used for iv
 option (USE_QT "Use Qt if found" ON)