Add included headers needed by flint 3.0.1

Also replace the manually specified CFLAGS and LDFLAGS with ones returned by
pkg-config.

There is a bug in flint's .pc file where -lflint is missing, so keep manually
adding that flag.

solver/solver.go

@@ -1,11 +1,15 @@
 package solver
 
 /*
-#cgo CFLAGS: -I/usr/local/include
-#cgo LDFLAGS: -L/usr/local/lib -lflint -lmpfr -lgmp -lm
+#cgo pkg-config: flint
+// missing from flint.pc as of 3.0.1
+#cgo LDFLAGS: -lflint
 #include <stdlib.h>
 #include <flint/flint.h>
+#include <flint/fmpz.h>
+#include <flint/fmpz_mod.h>
 #include <flint/fmpz_mod_poly.h>
+#include <flint/fmpz_mod_poly_factor.h>
 */
 import "C"
 import (