Handle -i{system,quote} without space before path

[dobo90]

• Although manual doesn't specify this it is allowed to pass -isystem or -iquote like this: -isystem/opt/lib

• If space is present before path, keep the current logic.

• Simple test (checked with gcc 9.1.0 and 5.3.0):

  mkdir -p /tmp/lib touch /tmp/lib/inc.h

  cat <<EOF > test.c #include "inc.h"

  int main(void) { return 0; } EOF

  gcc -isystem/tmp/lib test.c -o test