configure: rename have_incompatible_fork_abi -> have_incompatible_libav_abi

"Fork" is an ambiguous term, since there may be more than one fork, also
"fork" could be confused with the corresponding function. Also it seems
important to point the name of the referenced fork in the symbol.

Note: the old --enable-incompatible-fork-abi command is retained for
compatibility, although it should be considered deprecated.

configure

@@ -102,6 +102,8 @@ Configuration options:
   --enable-gray            enable full grayscale support (slower color)
   --disable-swscale-alpha  disable alpha channel support in swscale
   --disable-all            disable building components, libraries and programs
+  --enable-incompatible-libav-abi enable incompatible Libav fork ABI [no]
+  --enable-incompatible-fork-abi  enable incompatible Libav fork ABI (deprecated) [no]
 
 Program options:
   --disable-programs       do not build command line programs
@@ -1259,6 +1261,7 @@ CONFIG_LIST="
     gpl
     gray
     hardcoded_tables
+    incompatible_libav_abi
     incompatible_fork_abi
     lsp
     lzo
@@ -1365,6 +1368,7 @@ HAVE_LIST_CMDLINE='
 HAVE_LIST_PUB='
     bigendian
     fast_unaligned
+    incompatible_libav_abi
     incompatible_fork_abi
 '
 
@@ -3700,6 +3704,10 @@ EOF
     exit 1;
 fi
 
+# backward compatibility layer for incompatible_libav/fork_abi
+enabled incompatible_fork_abi  && enable incompatible_libav_abi
+enabled incompatible_libav_abi && enable incompatible_fork_abi
+
 die_license_disabled() {
     enabled $1 || { enabled $2 && die "$2 is $1 and --enable-$1 is not specified."; }
 }