It would seem a recent patch on 4.2's include/allegro/alcompat.h introduced in revision 9616 makes allegro unable to compile(at least on my machine) under linux.
If I 'update' that file to an earlier revision, all is well...
dev@xxxxxxxxxx ~/allegro-4.2-svn $ make gcc -DALLEGRO_MODULES_PATH=\"/usr/local/lib/allegro\" -DHAVE_CONFIG_H -I. -Iinclude -Iinclude/allegro -I./include -I./include/allegro -DALLEGRO_LIB_BUILD -mtune=pentium -O2 -funroll-loops -ffast-math -fomit-frame-pointer -Wall -Wno-unused -fPIC -DALLEGRO_SHARED -c ./src/allegro.c -o obj/unix/shared/alleg/allegro.o
In file included from include/allegro.h:66, from ./src/allegro.c:23: include/allegro/alcompat.h: In function 'get_file_encoding': include/allegro/alcompat.h:231: error: 'encoding' undeclared (first use in this function)
include/allegro/alcompat.h:231: error: (Each undeclared identifier is reported only once include/allegro/alcompat.h:231: error: for each function it appears in.) include/allegro/alcompat.h:231: error: too many arguments to function 'get_filename_encoding'
make: *** [obj/unix/shared/alleg/allegro.o] Error 1