configure: add GPLv2 support (needed for ProRes)

Signed-off-by: Elvis Presley <elvis@e.p>

configure

@@ -76,6 +76,7 @@ Configuration options:
   --enable-shared          build shared libraries [no]
   --enable-gpl             allow use of GPL code, the resulting libs
                            and binaries will be under GPL [no]
+  --enable-version2        force GPL version 2 [no]
   --enable-version3        upgrade (L)GPL to version 3 [no]
   --enable-nonfree         allow use of nonfree code, the resulting libs
                            and binaries will be unredistributable [no]
@@ -1039,6 +1040,7 @@ CONFIG_LIST="
     thumb
     vaapi
     vdpau
+    version2
     version3
     x11grab
     zlib
@@ -1205,6 +1207,7 @@ HAVE_LIST="
 CONFIG_EXTRA="
     avutil
     gplv3
+    gplv2
     lgplv3
 "
 
@@ -2676,7 +2679,11 @@ die_license_disabled version3 libopencore_amrwb
 die_license_disabled version3 libvo_aacenc
 die_license_disabled version3 libvo_amrwbenc
 
+! enabled gpl && enabled version2 && die "LGPLv2 mode is unsupported"
+enabled version3 && enabled version2 && ! enabled nonfree && die "GPLv2 (not v2+) and GPLv3 without --enable-nonfree"
+
 enabled version3 && { enabled gpl && enable gplv3 || enable lgplv3; }
+enabled version2 && enabled gpl && enable gplv2
 
 disabled optimizations || check_cflags -fomit-frame-pointer
 
@@ -3345,6 +3352,8 @@ if enabled nonfree; then
     license="nonfree and unredistributable"
 elif enabled gplv3; then
     license="GPL version 3 or later"
+elif enabled gplv2; then
+    license="GPL version 2"
 elif enabled lgplv3; then
     license="LGPL version 3 or later"
 elif enabled gpl; then