microseconds.c:
#include <time.h>
unsigned long current_microseconds(void) {
  struct timespec ts;
  clock_gettime(CLOCK_MONOTONIC, &ts);
  return (unsigned long)(1e6*ts.tv_sec + ts.tv_nsec/1e3);
}

microseconds.scm:
#>
extern unsigned long current_microseconds(void);
<#
(define current-microseconds (foreign-lambda unsigned-long "current_microseconds"))

bash:
gcc -Wall -Wextra -O3 -pedantic -std=gnu99 -c microseconds.c
csc -s microseconds.c microseconds.scm

csi:
#;1> (load "microseconds.so")
; loading microseconds.so ...
#;2> (current-microseconds)
40677968507