@@ -23,13 +23,25 @@
 # TODO: make it possible for user to change
 #       default set with environment vars
 modules=(kcore kmem kstr kio kgraft kfile)
 
+# compose an arch tuple. this is used in
+# places, mostly to select the correct
+# version of assembly files for a given
+# platform (each has an arch tuple in its
+# name, following the linkage specifier)
 target=$arch.$os
 if test "$bits" != ""; then
 	target=$target.$bits
 fi
 
+# determine whether we have unix or posix
+# APIs depending on the operating system.
+# presumably if the user is running a bash
+# script there is some degree of posix
+# support available, but we might still be
+# building for windows from within cygwin
+# or whatever
 case $os in
 	lin|?bsd|and|dar|osx) posix=yes; unix=yes;;
 	hai) posix=yes; unix=no;;
 	*) posix=no; unix=no;;
@@ -81,15 +93,36 @@
 check m4 "the path to your m4 installation"
 
 export build=$(global/build-id.sh)
 
-case $os in
-	lin) p_headers_syscall=${p_headers_syscall:-/usr/include/asm/unistd_${bits}.h}
-	p_headers_errno=${p_headers_errno:-/usr/include/asm-generic/errno.h /usr/include/asm-generic/errno-base.h};;
+if test "$p_headers_syscall" = ""; then
+	case $os in
+		lin) headers_syscall_search=(
+				/usr/include/asm/unistd_${bits}.h
+				/usr/include/asm-generic/unistd.h
+				/usr/include/*-linux-gnu/asm/unistd_${bits}.h
+			);;
 
-	fbsd) p_headers_syscall=${p_headers_syscall:-/usr/include/sys/syscall.h}
-	p_headers_errno=${p_headers_errno:-/usr/include/errno.h};;
-esac
+		fbsd) p_headers_syscall_search=(
+				/usr/include/sys/syscall.h
+			);;
+	esac
+fi
+
+if test "$p_headers_errno" = ""; then
+	case $os in
+		lin) p_headers_errno="${p_headers_errno:-/usr/include/asm-generic/errno.h /usr/include/asm-generic/errno-base.h}";;
+
+		fbsd) p_headers_errno="${p_headers_errno:-/usr/include/errno.h}";;
+	esac
+fi
+
+for f in "${headers_syscall_search[@]}"; do
+	test -e "$f" || continue
+	p_headers_syscall="$f"
+	say "using syscall headers at $f"
+	break;
+done
 
 check p_headers_syscall \
 	'the location of a header defining your syscall numbers'
 check p_headers_errno \