This will translate a file that consists of backslash-newline broken lines into a file with single long lines, and squash whitespace.
For example:
acpi_quirks.h optional acpi \ dependency "$S/tools/acpi_quirks2h.awk $S/dev/acpica/acpi_quirks" \ compile-with "${AWK} -f $S/tools/acpi_quirks2h.awk $S/dev/acpica/acpi_quirks" \ no-obj no-implicit-rule before-depend \ clean "acpi_quirks.h"
becomes
acpi_quirks.h optional acpi dependency "$S/tools/acpi_quirks2h.awk $S/dev/acpica/acpi_quirks" compile-with "${AWK} -f $S/tools/acpi_quirks2h.awk $S/dev/acpica/acpi_quirks" no-obj no-implicit-rule before-depend clean "acpi_quirks.h"
expand "${file}" | sed -ne ' /[^\\]$/ { H s/.*//g x s/^\n//g s/\n/ /g s/ */ /g p } /\\$/ { s/\\$// H }'