Remove dependency on GMP, so that it is easier to run the bench suite.

tests/Makefile

@@ -1,10 +1,9 @@
-#NOTE: Neither GMP not pthread is not a real dependency. It is only used for the test suite.
 CC=cc -std=c99
 WCFLAGS=-Wall -W -pedantic -Wshadow -Wpointer-arith -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -Wswitch-default -Wswitch-enum -Wcast-align -Wpointer-arith -Wbad-function-cast -Wstrict-overflow=5 -Wstrict-prototypes -Winline -Wundef -Wnested-externs -Wcast-qual -Wshadow -Wunreachable-code -Wlogical-op -Wstrict-aliasing=2 -Wredundant-decls -Wold-style-definition -Wno-unused-function -ftrapv
 XCFLAGS=
 CFLAGS=$(WCFLAGS) $(XCFLAGS)
 CPPFLAGS=-I..
-LDFLAGS=-pthread -lgmp
+LDFLAGS=-pthread
 RM=rm -f
 
 all: