[bug-libmatheval] libmatheval finds -ll for FLEXLIB, but tries to use -lfl

[Hanspeter Niederstrasser]

On OS X 10.8, libmatheval fails to build if only the system flex 
(2.5.35) is installed.  /usr/lib has libl.a, but not libfl.a

This is the error:

libtool: link: gcc -dynamiclib -Wl,-undefined -Wl,dynamic_lookup -o 
.libs/libmatheval.1.dylib  .libs/parser.o .libs/scanner.o .libs/error.o 
.libs/matheval.o .libs/g77_interface.o .libs/node.o .libs/symbol_table.o 
.libs/xmalloc.o .libs/xmath.o   -L/sw/lib -lfl -lm  -O2   -install_name 
 /sw/lib/libmatheval.1.dylib -compatibility_version 2 -current_version 
2.0 -Wl,-single_module
ld: library not found for -lfl

./configure correctly detects -ll and saves LEXLIB='-ll'

However, lib/Makefile.am hardcodes '-lfl' and applies that to the linker 
command.  The following simple patch uses an automake substitution token 
to avoid hardcoding the lex library name.

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--- lib/Makefile.am.orig        2012-11-29 06:46:16.000000000 -0500
+++ lib/Makefile.am     2013-08-13 16:42:43.000000000 -0400
@@ -29,7 +29,7 @@
 g77_interface.c node.c symbol_table.c xmalloc.c xmath.c

 libmatheval_la_LDFLAGS = -version-info 1:0:0
-libmatheval_la_LIBADD = -lfl -lm
+libmatheval_la_LIBADD = @LEXLIB@ -lm

 include_HEADERS = matheval.h
 noinst_HEADERS = common.h error.h node.h symbol_table.h xmalloc.h      \

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Hanspeter