$ make -k
...
gcc  -I. -fasynchronous-unwind-tables -fexceptions -fstack-protector-strong -Wer
ror=implicit-function-declaration -fdata-sections -ffunction-sections -fPIC  -Wa
,--noexecstack -g -O2 -c ./popen3.c
./popen3.c: In function 'popen3':
./popen3.c:106: error: 'fd_set' undeclared (first use in this function)
./popen3.c:106: error: (Each undeclared identifier is reported only once
./popen3.c:106: error: for each function it appears in.)
./popen3.c:106: error: expected ';' before 'rfds'
./popen3.c:111: warning: implicit declaration of function 'FD_ZERO'
./popen3.c:111: error: 'rfds' undeclared (first use in this function)
./popen3.c:112: warning: implicit declaration of function 'FD_SET'
./popen3.c:113: warning: implicit declaration of function 'select'