Set target_path to $(PWD) by default

This simplifies configuring for the most common remote testing
setup.

Originally committed as revision 21525 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1449,7 +1449,7 @@ CC_O='-o $@'
 host_cflags='-D_ISOC99_SOURCE -D_POSIX_C_SOURCE=200112 -O3 -g -Wall'
 host_libs='-lm'
 
-target_path='.'
+target_path='$(PWD)'
 
 # gcc stupidly only outputs the basename of targets with -MM, but we need the
 # full relative path for objects in subdirectories for non-recursive Make.