fontconfig is easy

example.c

#include <fontconfig/fontconfig.h>
#include <stdio.h>

int main(int argc, char** argv) {
  FcInit();

  for (int i=1; i<argc; i++) {
    FcChar8* lower = FcStrDowncase(argv[i]);
    printf("%s\n", lower);
    FcStrFree(lower);
  }
}

Makefile

CFLAGS=$(shell pkg-config --cflags fontconfig)
LDFLAGS=$(shell pkg-config --libs fontconfig)

example: example.o