ffmpeg: fix static linking build failure when using libavutil

If a package tries to static link with libavutil it fails due to the
lack of libavutil private dependencies in libavutil.pc (-ldrm in this
case).

Add patch to:
- Check if libdrm is present.
- Add it to Libs.private: in libavutil.pc if present.

Fixes:
http://autobuild.buildroot.net/results/766/766de487f394490df8c712652ac364ebb4a3ab14/
http://autobuild.buildroot.net/results/041/041e29dfddb2da3309ac7d34a576c60c5a75fe4d/
http://autobuild.buildroot.net/results/780/78061b61cfe3f42554a475c048d54dacacfe11d5/
http://autobuild.buildroot.net/results/275/275e4e0030d26c029085b408cfb272d5633969c6/
http://autobuild.buildroot.net/results/515/5152dcca58944cf732d09fba6e6c9af8a9243c75/
http://autobuild.buildroot.net/results/395/395be1a9cab824b82ef34c2ebd84d54243029b33/

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/ffmpeg/0002-configure-add-LIBDRM-to-Libs.private.patch

@@ -0,0 +1,68 @@
+From e1e11d404eec24c158ae1241d6573c263e57a7b9 Mon Sep 17 00:00:00 2001
+From: Giulio Benetti <giulio.benetti@micronovasrl.com>
+Date: Wed, 17 Oct 2018 13:13:10 +0200
+Subject: [PATCH] configure: add LIBDRM to Libs.private
+
+When static linking programs using ffmpeg libraries, if linking against
+libavutil, -ldrm is listed before -lavutil. This leads to linking
+failure due to undefined reference of drmGetVersion() and
+drmFreeVersion().
+This is why when pkg-config create libavutil.pc doesn't append -ldrm
+to Libs.private:.
+
+- Create LIBDRM=-ldrm in case libdrm is enabled.
+- Add privatelibs_avutil variable for explicit private libraries to be
+  appended to Libs.private:
+- Add $LIBDRM to privatelibs_avutil.
+
+Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
+---
+ configure                     | 3 ++-
+ ffbuild/pkgconfig_generate.sh | 3 ++-
+ 2 files changed, 4 insertions(+), 2 deletions(-)
+
+diff --git a/configure b/configure
+index 7377046d0a..0996d8a85d 100755
+--- a/configure
++++ b/configure
+@@ -5919,7 +5919,7 @@ enabled libcelt           && require libcelt celt/celt.h celt_decode -lcelt0 &&
+                                die "ERROR: libcelt must be installed and version must be >= 0.11.0."; }
+ enabled libcaca           && require_pkg_config libcaca caca caca.h caca_create_canvas
+ enabled libdc1394         && require_pkg_config libdc1394 libdc1394-2 dc1394/dc1394.h dc1394_new
+-enabled libdrm            && require_pkg_config libdrm libdrm xf86drm.h drmGetVersion
++enabled libdrm            && require_pkg_config libdrm libdrm xf86drm.h drmGetVersion && LIBDRM="-ldrm"
+ enabled libfdk_aac        && { use_pkg_config libfdk_aac fdk-aac "fdk-aac/aacenc_lib.h" aacEncOpen ||
+                                { require libfdk_aac fdk-aac/aacenc_lib.h aacEncOpen -lfdk-aac &&
+                                  warn "using libfdk without pkg-config"; } }
+@@ -7097,6 +7097,7 @@ LIBPREF=${LIBPREF}
+ LIBSUF=${LIBSUF}
+ 
+ extralibs_avutil="$LIBRT $LIBM"
++privatelibs_avutil="$LIBDRM"
+ extralibs_avcodec="$extralibs"
+ extralibs_avformat="$extralibs"
+ extralibs_avdevice="$extralibs"
+diff --git a/ffbuild/pkgconfig_generate.sh b/ffbuild/pkgconfig_generate.sh
+index e5de6716d2..b86486a107 100755
+--- a/ffbuild/pkgconfig_generate.sh
++++ b/ffbuild/pkgconfig_generate.sh
+@@ -13,6 +13,7 @@ name=lib${shortname}
+ fullname=${name}${build_suffix}
+ comment=$2
+ libs=$(eval echo \$extralibs_${shortname})
++privatelibs=$(eval echo \$privatelibs_${shortname})
+ deps=$(eval echo \$${shortname}_deps)
+ 
+ for dep in $deps; do
+@@ -39,7 +40,7 @@ Requires: $($shared || echo $requires)
+ Requires.private: $($shared && echo $requires)
+ Conflicts:
+ Libs: -L\${libdir} $rpath -l${fullname#lib} $($shared || echo $libs)
+-Libs.private: $($shared && echo $libs)
++Libs.private: $($shared && echo $libs) ${privatelibs}
+ Cflags: -I\${includedir}
+ EOF
+ 
+-- 
+2.17.1
+