[BUILD] Replace hardcoded 'LD = gcc' with 'LD = $(CC)'

haproxy relies on linking the binary using gcc, so there is no real need to
hardcode both (CC and LD). Setting 'LD = $(CC)' will make the build system
a bit more cross-compile friendly because only the right cross-compiler has
to be passed via make.

Makefile

@@ -81,7 +81,7 @@ CPU = generic
 #### Toolchain options.
 # GCC is normally used both for compiling and linking.
 CC = gcc
-LD = gcc
+LD = $(CC)
 
 #### Debug flags (typically "-g").
 # Those flags only feed CFLAGS so it is not mandatory to use this form.