Rewrite the public symbol check to verify the shared libraries, to check for
more things, and to avoid duplication; fixes make check on ARM

Taken from Debian:
https://salsa.debian.org/multimedia-team/mpeg2dec/blob/master/debian/patches/61_global-symbol-test.patch

--- mpeg2dec.orig/test/globals
+++ mpeg2dec/test/globals
@@ -1,4 +1,8 @@
 #!/bin/sh
+# TODO
+# - fix checking of .a libs; problem is that "nm -g --defined-only" lists
+#   internal symbols; this can be solved by using objdump, but it's probably
+#   good enough to just run the tests on the shared lib
 
 if test x"$srcdir" != x""; then
     builddir="."	# running from make check, but it does not define that
@@ -14,22 +18,30 @@ builddir=`cd $builddir;pwd`
 
 error=0
 
-bad_globals=`nm -g --defined-only $builddir/../libmpeg2/*.o |\
-    awk '{if ($3) print $3}' | grep -v '^_\?mpeg2_'`
-
-if test x"$bad_globals" != x""; then
-    echo BAD GLOBAL SYMBOLS:
-    for s in $bad_globals; do echo $s; done
+# check_bad_public_symbols <symbol prefix> <lib file> [<lib file>...]
+#
+# checks public symbols in shared libs:
+# - allow prefix_anything
+# - reject _prefixanything
+# - allow _anything
+# - reject anything else
+#
+# NB: skips missing files
+check_bad_public_symbols() {
+    symbols_prefix="$1"
+    shift
+    lib_files=`ls "$@" 2>/dev/null`
+    [ -z "$lib_files" ] && return
+    bad_globals=`nm -g --defined-only $lib_files |
+        awk '{if ($3) print $3}' |
+        sed -n "/^${symbols_prefix}_/ d; /^_${symbols_prefix}/ { p; d }; /^_/ d; p"`
+    [ -z "$bad_globals" ] && return
     error=1
-fi
-
-bad_globals=`nm -g --defined-only $builddir/../libmpeg2/convert/*.o |\
-    awk '{if ($3) print $3}' | grep -v '^_\?mpeg2convert_'`
+    echo BAD GLOBAL SYMBOLS in $lib_files:
+    echo "$bad_globals"
+}
 
-if test x"$bad_globals" != x""; then
-    echo BAD GLOBAL SYMBOLS:
-    for s in $bad_globals; do echo $s; done
-    error=1
-fi
+check_bad_public_symbols mpeg2 $builddir/../libmpeg2/.libs/libmpeg2.so
+check_bad_public_symbols mpeg2convert $builddir/../libmpeg2/convert/.libs/libmpeg2convert.so
 
 exit $error