Revert "default build without GMP"

This reverts commit 8e986394ae.

common.mk

@@ -110,7 +110,7 @@ ifeq ($(DEBUG),0)
 CFLAGS+=$(CFLAGS_OPT_USER)
 endif
 CFLAGS+=$(CFLAGS_USER)
-MCL_USE_GMP?=0
+MCL_USE_GMP?=1
 ifeq ($(OS),mac)
   ifeq ($(shell sw_vers -productVersion),10.15)
     # workaround because of GMP does not run well on Catalina

readme.md

@@ -51,6 +51,14 @@ see [api.md](api.md)
 # How to build on Linux and macOS
 x86-64/ARM/ARM64 Linux, macOS and mingw64 are supported.
 
+## Installation Requirements
+
+[GMP](https://gmplib.org/) is necessary (default setting).
+
+```
+apt install libgmp-dev # on Ubuntu
+```
+
 ## How to build with Makefile
 
 ```
@@ -72,15 +80,10 @@ make bin/bn_c384_256_test.exe && bin/bn_c384_256_test.exe
 make bin/bls12_test.exe && bin/bls12_test.exe
 ```
 
-## How to build with GMP
-Install [GMP](https://gmplib.org/).
-
-```
-apt install libgmp-dev # on Ubuntu
-```
+## How to build without GMP
 
 ```
-make MCL_USE_GMP=1
+make MCL_USE_GMP=0
 
 ```
 Define `MCL_USE_VINT` if using C++ header files.