[libpng16] Attempt to fix problem with autogen.sh

Renamed internal configure variable from prefix_TRUE to DO_PNG_PREFIX_TRUE

Makefile.am

@@ -121,7 +121,7 @@ libpng.vers: scripts/vers.out
 	rm -f $@
 	cp $? $@
 
-if prefix
+if DO_PNG_PREFIX
 # Rename functions in scripts/prefix.out with a PNG_PREFIX prefix.
 # Rename macros in scripts/macro.lst from PNG_PREFIXpng_ to PNG_ (the actual
 # implementation of the macro).
@@ -167,7 +167,7 @@ SYMBOL_CFLAGS = -DPNGLIB_LIBNAME='PNG@PNGLIB_MAJOR@@PNGLIB_MINOR@_0'\
 		-DSYMBOL_PREFIX='$(SYMBOL_PREFIX)'\
 		-DPNG_NO_USE_READ_MACROS -DPNG_BUILDING_SYMBOL_TABLE
 
-if prefix
+if DO_PNG_PREFIX
 SYMBOL_CFLAGS += -DPNG_PREFIX='@PNG_PREFIX@'
 endif
 

configure.ac

@@ -231,7 +231,7 @@ AC_ARG_WITH(libpng-prefix,
    [if test "${withval:-no}" != "no"; then
       AC_SUBST([PNG_PREFIX], [${withval}])
     fi])
-AM_CONDITIONAL([prefix], [test "${with_libpng_prefix:-no}" != "no"])
+AM_CONDITIONAL([DO_PNG_PREFIX], [test "${with_libpng_prefix:-no}" != "no"])
 
 # Because GCC by default assembles code with an executable stack, even though it
 # compiles C code with a non-executable stack, it is necessary to do a fixup