add support for hosts that have custom symbol prefixes

From: Mike Frysinger <vapier@gentoo.org>

some architectures, like Blackfin, have a prefix added to all symbols (in our
case it's historical baggage)

while normally packages shouldnt care, when code starts mixing assembler with
C (like symbol versioning), you need to dip down into the inner details.
find attached a patch which queries gcc for the prefix (all gcc toolchains
have a builtin define of __USER_LABEL_PREFIX__) and then automatically has
asm() constructs utilize this

since most targets define __USER_LABEL_PREFIX__ to nothing, hopefully this
shouldnt cause any problems :)

configure.in

@@ -34,6 +34,7 @@ fi
 	    
 
 AC_PROG_CC
+AC_PROG_CPP
 AC_PROG_INSTALL
 AC_PROG_LN_S 
 AC_DISABLE_STATIC
@@ -105,6 +106,16 @@ else
 fi
 AM_CONDITIONAL(VERSIONED_SYMBOLS, test x$versioned = xyes)
 
+dnl See if toolchain has a custom prefix for symbols ...
+AC_MSG_CHECKING(for custom symbol prefixes)
+SYMBOL_PREFIX=` \
+	echo "PREFIX=__USER_LABEL_PREFIX__" \
+		| ${CPP-${CC-gcc} -E} - 2>&1 \
+		| ${EGREP-grep} "^PREFIX=" \
+		| ${SED-sed} "s:^PREFIX=::"`
+AC_DEFINE_UNQUOTED([__SYMBOL_PREFIX], "$SYMBOL_PREFIX", [Toolchain Symbol Prefix])
+AC_MSG_RESULT($SYMBOL_PREFIX)
+
 dnl Check for debug...
 AC_MSG_CHECKING(for debug)
 AC_ARG_WITH(debug,