mythtv/configure: fix detection of FFmpeg's version numbers

FFmpeg has split their version.h into version_major.h and version.h.

libavutil has an empty version_major.h.

mythtv/configure

@@ -7750,7 +7750,8 @@ get_version(){
     lcname=lib${1}
     name=$(toupper $lcname)
     file=$source_path/external/FFmpeg/$lcname/version.h
-    eval $(awk "/#define ${name}_VERSION_M/ { print \$2 \"=\" \$3 }" "$file")
+    file_major=$source_path/external/FFmpeg/$lcname/version_major.h
+    eval $(awk "/#define ${name}_VERSION_M/ { print \$2 \"=\" \$3 }" "$file_major" "$file")
     enabled raise_major && eval ${name}_VERSION_MAJOR=$((${name}_VERSION_MAJOR+100))
     eval ${name}_VERSION=\$${name}_VERSION_MAJOR.\$${name}_VERSION_MINOR.\$${name}_VERSION_MICRO
     eval echo "${lcname}_VERSION=\$${name}_VERSION" >> $TMPMAK