# which gp
/usr/local/bin/gp 

# which pari
which: no pari in (/sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/X11R6/bin:/usr/games:/usr/lib64/jvm/jre/bin:/usr/lib/mit/bin:/usr/lib/mit/sbin)

# perl Makefile.PL paridir=/usr/local/bin/gp
Setting up Math::Pari with Perl 5.010000 on linux version 2.6.31;
   cc=cc, gccversion=4.4.1 [gcc-4_4-branch revision 150839], cccdlflags='-fPIC',
   ccflags='-D_REENTRANT -D_GNU_SOURCE -DPERL_USE_SAFE_PUTENV -DDEBUGGING -fno-strict-aliasing -pipe -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
   optimize='-fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -Wall -pipe',
   ld=cc, ldflags=' -L/usr/local/lib64'.
Could not extract version from '/usr/local/bin/gp/config/version';

   Do you remember that paridir should be location of PARI/GP source tree,
   not the location of compiled+installed files?

Trying extract from the directory name...
Directory `/usr/local/bin/gp' has unknown syntax...
Found GP/PARI build directory in /usr/local/bin/gp
     (integer-formatted version /usr/local/bin/gp).
Cannot find tests in /usr/local/bin/gp/src/test/in: Not a directory at utils/Math/PariBuild.pm line 618