Add -D_GNU_SOURCE to CFLAGS for program_invocation_short_name()

diff -upk.orig bzip2-1.0.4.orig/bzip2.c bzip2-1.0.4/bzip2.c
--- bzip2-1.0.4.orig/bzip2.c	2007-01-10 22:05:04 +0000
+++ bzip2-1.0.4/bzip2.c	2007-01-10 22:06:38 +0000
@@ -209,8 +209,7 @@ Int32   longestFileName;
 Char    inName [FILE_NAME_LEN];
 Char    outName[FILE_NAME_LEN];
 Char    tmpName[FILE_NAME_LEN];
-Char    *progName;
-Char    progNameReally[FILE_NAME_LEN];
+const char    *progName;
 FILE    *outputHandleJustInCase;
 Int32   workFactor;
 
@@ -1795,7 +1794,6 @@ void addFlagsFromEnvVar ( Cell** argList
 IntNative main ( IntNative argc, Char *argv[] )
 {
    Int32  i, j;
-   Char   *tmp;
    Cell   *argList;
    Cell   *aa;
    Bool   decode;
@@ -1834,10 +1832,7 @@ IntNative main ( IntNative argc, Char *a
    copyFileName ( inName,  (Char*)"(none)" );
    copyFileName ( outName, (Char*)"(none)" );
 
-   copyFileName ( progNameReally, argv[0] );
-   progName = &progNameReally[0];
-   for (tmp = &progNameReally[0]; *tmp != '\0'; tmp++)
-      if (*tmp == PATH_SEP) progName = tmp + 1;
+   progName = program_invocation_short_name;
 
 
    /*-- Copy flags from env var BZIP2, and 
diff -upk.orig bzip2-1.0.4.orig/bzip2recover.c bzip2-1.0.4/bzip2recover.c
--- bzip2-1.0.4.orig/bzip2recover.c	2007-01-10 22:05:04 +0000
+++ bzip2-1.0.4/bzip2recover.c	2007-01-10 22:06:38 +0000
@@ -67,7 +67,7 @@ typedef  unsigned char  Bool;
 
 Char inFileName[BZ_MAX_FILENAME];
 Char outFileName[BZ_MAX_FILENAME];
-Char progName[BZ_MAX_FILENAME];
+const char *progName;
 
 MaybeUInt64 bytesOut = 0;
 MaybeUInt64 bytesIn  = 0;
@@ -333,7 +333,7 @@ Int32 main ( Int32 argc, Char** argv )
    UInt32      buffHi, buffLo, blockCRC;
    Char*       p;
 
-   strcpy ( progName, argv[0] );
+   progName = program_invocation_short_name;
    inFileName[0] = outFileName[0] = 0;
 
    fprintf ( stderr,