build: Avoid absolute paths during "whole-program" compiling.

The build currently does a textual include of all files in order to
use the -fwhole-compile optimization.  Update it to use relative file
paths instead of absolute file paths.  This makes the section names in
the resulting binary more readable.  It also makes the build easier on
some Windows hosts.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/Makefile b/Makefile
index fb4e683..edc8d8d 100644
--- a/Makefile
+++ b/Makefile
@@ -111,8 +111,8 @@
 # Do a whole file compile by textually including all C code.
 define whole-compile
 @echo "  Compiling whole program $3"
-$(Q)printf '$(foreach i,$2,#include "$(CURDIR)/$i"\n)' > $3.tmp.c
-$(Q)$(CC) $1 $(CFLAGSWHOLE) -c $3.tmp.c -o $3
+$(Q)printf '$(foreach i,$2,#include "$i"\n)' > $3.tmp.c
+$(Q)$(CC) -I. $1 $(CFLAGSWHOLE) -c $3.tmp.c -o $3
 endef
 
 %.strip.o: %.o