wmacpi: Remove deprecated /usr/X11R6/include directory from CFLAGS.
wmacpi: Merge clean and clean-all targets in Makefile.

"make clean" should do the expected thing.
wmacpi: Respect DESTDIR variable in Makefile, e.g., for package creation.
wmacpi: Fix -Wunused-but-set-variable compiler warning.

In particular,
libacpi.c: In function ‘procfs_get_battery_info’:
libacpi.c:582:9: warning: variable ‘buflen’ set but not used [-Wunused-but-set-variable]
     int buflen;
         ^
wmacpi: Fix -Wunused-result compiler warnings.

In particular,
libacpi.c: In function ‘power_init’:
libacpi.c:339:5: warning: ignoring return value of ‘fread’, declared with attribute warn_unused_result [-Wunused-result]
     fread(buf, 4096, 1, acpi);
     ^
libacpi.c: In function ‘procfs_get_power_status’:
libacpi.c:434:5: warning: ignoring return value of ‘fgets’, declared with attribute warn_unused_result [-Wunused-result]
     fgets(buf, 1024, file);
     ^
wmacpi: Bump to version 2.3.