We put macros in a response file compile_time_defs.txt and pass it
to the compiler. Adding a pair of single quotes around each -D flag
ensures macro values are quoted correctly.

For example,
* String
  * target_compile_definitions(): either FOO="BAR" or FOO=\"BAR\"
  * response file: '-DFOO="BAR"'
  * actual definition: #define FOO "BAR"
* Array of integers
  * target_compile_definitions(): FOO={1, 2, 3}
  * response file: '-DFOO={1, 2, 3}'
  * actual definition: #define FOO {1, 2, 3}