Add dependency file generation to build (#644)

Fixes #643

Author: earlephilhower

platform.txt

@@ -48,13 +48,13 @@ compiler.wrap="@{runtime.platform.path}/lib/platform_wrap.txt"
 compiler.libpico="{runtime.platform.path}/lib/libpico.a"
 
 compiler.c.cmd=arm-none-eabi-gcc
-compiler.c.flags=-c {compiler.warning_flags} {compiler.defines} {compiler.flags} {compiler.includes} -std=gnu17 -g
+compiler.c.flags=-c {compiler.warning_flags} {compiler.defines} {compiler.flags} -MMD {compiler.includes} -std=gnu17 -g
 compiler.c.elf.cmd=arm-none-eabi-g++
 compiler.c.elf.flags={compiler.warning_flags} {compiler.defines} {compiler.flags} {build.flags.optimize} -u _printf_float -u _scanf_float
 compiler.S.cmd=arm-none-eabi-gcc
 compiler.S.flags=-c {compiler.warning_flags} -g -x assembler-with-cpp -MMD {compiler.includes} -g
 compiler.cpp.cmd=arm-none-eabi-g++
-compiler.cpp.flags=-c {compiler.warning_flags} {compiler.defines} {compiler.flags} {compiler.includes} {build.flags.rtti} -std=gnu++17 -g
+compiler.cpp.flags=-c {compiler.warning_flags} {compiler.defines} {compiler.flags} -MMD {compiler.includes} {build.flags.rtti} -std=gnu++17 -g
 
 compiler.ar.cmd=arm-none-eabi-ar
 compiler.ar.flags=rcs