make install prepends DESTDIR to custom libdir/bindir path

[Danny]

Summary of the bug:

Specifying a custom install directory in ./configure prepends the environment variable DESTDIR to the path.

This behavior should be diabled or otherwise documented in "./configure --help"

How to reproduce:
With this configure line:

% cd ffmpeg
./configure \
--extra-ldflags="-L/mnt/swdevel/myProject/lib3p$LIBSUFFIX -L${ffmpeg_exported_release_dir}/lib"  \
--extra-cflags="-I/mnt/swdevel/myProject/include -I${ffmpeg_exported_release_dir}/include" \
--disable-static --enable-shared  --disable-cuda --disable-cuvid \
--disable-nvenc  --enable-libx264 --enable-gpl --enable-libfreetype --enable-libfontconfig \
--enable-libv4l2 --enable-indev=alsa --enable-indev=v4l2 \
--bindir=/mnt/swdevel/myProject/bin$LIBSUFFIX \
--datadir=/mnt/swdevel/myProject/ext/ffmpeg-build \
--docdir=/mnt/swdevel/myProject/ext/doc \
--libdir=/mnt/swdevel/myProject/lib3p$LIBSUFFIX \
--shlibdir=/mnt/swdevel/myProject/lib3p$LIBSUFFIX \
--incdir=/mnt/swdevel/myProject/ext/extinclude/ffmpeg \
--mandir=/mnt/swdevel/myProject/ext/ffmpeg-build$LIBSUFFIX \
--enable-libxcoder --disable-doc --enable-x86asm \
--extra-ldflags=-lm --enable-pthreads --extra-libs=-lpthread \
--disable-debug

With DESTDIR set to /mnt/swdevel/myProject

$ make install V=1
mkdir -p "/mnt/swdevel/myProject/mnt/swdevel/myProject/lib3p64"
install -m 755 libavdevice/libavdevice.so "/mnt/swdevel/myProject/mnt/swdevel/myProject/lib3p64/libavdevice.so.58.13.100"
strip "/mnt/swdevel/myProject/mnt/swdevel/myProject/lib3p64/libavdevice.so.58.13.100"
(cd "/mnt/swdevel/myProject/mnt/swdevel/myProject/lib3p64" && ln -s -f libavdevice.so.58.13.100 libavdevice.so.58); (cd "/mnt/swdevel/myProject/mnt/swdevel/myProject/lib3p64" && ln -s -f libavdevice.so.58.13.100 libavdevice.so);
mkdir -p "/mnt/swdevel/myProject/mnt/swdevel/myProject/lib3p64"
etc

After unset DESTDIR

$ make install V=1
mkdir -p "/mnt/swdevel/myProject/lib3p64"
install -m 755 libavdevice/libavdevice.so "/mnt/swdevel/myProject/lib3p64/libavdevice.so.58.13.100"
strip "/mnt/swdevel/myProject/lib3p64/libavdevice.so.58.13.100"
(cd "/mnt/swdevel/myProject/lib3p64" && ln -s -f libavdevice.so.58.13.100 libavdevice.so.58); (cd "/mnt/swdevel/myProject/lib3p64" && ln -s -f libavdevice.so.58.13.100 libavdevice.so);
mkdir -p "/mnt/swdevel/myProject/lib3p64"
install -m 755 libavfilter/libavfilter.so "/mnt/swdevel/myProject/lib3p64/libavfilter.so.7.110.100"

Compiling from branch n4.4.