Fix incorrectly named configure options

Although we should use 'AC_ARG_ENABLE' for boolean flags, it also
means options get named '--enable-*', not '--with-*'. This should
unbreak the --with-intree-gmp option.

aclocal.m4

@@ -1780,13 +1780,13 @@ AC_DEFUN([FP_GMP],
       [directory containing gmp library])],
       [GMP_LIB_DIRS=$withval])
 
-  AC_ARG_ENABLE([intree-gmp],
+  AC_ARG_WITH([intree-gmp],
     [AC_HELP_STRING([--with-intree-gmp],
       [force using the in-tree GMP])],
       [GMP_FORCE_INTREE=YES],
       [GMP_FORCE_INTREE=NO])
 
-  AC_ARG_ENABLE([gmp-framework-preferred],
+  AC_ARG_WITH([gmp-framework-preferred],
     [AC_HELP_STRING([--with-gmp-framework-preferred],
       [on OSX, prefer the GMP framework to the gmp lib])],
       [GMP_PREFER_FRAMEWORK=YES],

libraries/integer-gmp/configure.ac

@@ -27,13 +27,13 @@ AC_ARG_WITH([gmp-libraries],
     [GMP_LIB_DIRS=$withval; LDFLAGS="-L$withval"],
     [GMP_LIB_DIRS=])
 
-AC_ARG_ENABLE([gmp-framework-preferred],
+AC_ARG_WITH([gmp-framework-preferred],
   [AC_HELP_STRING([--with-gmp-framework-preferred],
     [on OSX, prefer the GMP framework to the gmp lib])],
     [GMP_PREFER_FRAMEWORK=YES],
     [GMP_PREFER_FRAMEWORK=NO])
 
-AC_ARG_ENABLE([intree-gmp],
+AC_ARG_WITH([intree-gmp],
   [AC_HELP_STRING([--with-intree-gmp],
     [force using the in-tree GMP])],
     [GMP_FORCE_INTREE=YES],