GMP and Readline detection is a bit off

Hi Everyone,

I'm working with the pari-2.13.0 source tarball. I have the latest GMP
6.2.1 and Readline 8.0 installed at prefix=/usr/local.

I noticed PARI/GP is not configuring correctly using:

    --with-gmp=/usr/local
    --with-readline=/usr/local

Here's what Configure is showing:

Checking for optional libraries and headers...
...Found gmp header in /usr/local/include
Using GNU MP, version 6.2.0
### Qt not found. Building without Qt support
Hi-Res Graphics: none
...Found readline header in /usr/local/include/readline
###
### Readline library does not seem to work. Maybe install libncurses?
###
### Building without GNU readline support

I installed GMP 6.2.1, but Configure is finding GMP 6.2.0. That's the
system's version of the library in /usr/lib64, not my version in
/usr/local/lib64.

I suspect the Configure test program is not using CPPFLAGS, CFLAGS and
LDFLAGS I supplied.

Jeff