pdfgrep: partially fix cross

This commit gets pkgsCross.*.pdfgrep a bit farther. It still fails
with the error below; if anybody recognizes it I'd appreciate a hint.

```
pdfgrep-aarch64-unknown-linux-gnu> aarch64-unknown-linux-gnu-g++ -DHAVE_CONFIG_H -I. -I.. -I/nix/store/9kzvsy9ybsy2zvivz62ia881q9djf734-poppler-glib-aarch64-unknown-linux-gnu-23.02.0-dev/include/poppler/cpp -I/nix/store/6ib0mcvv0ascsydhpja2a1kcnwvxqnfp-pcre-aarch64-unknown-linux-gnu-8.45-dev/include -I/nix/store/yyyy71mx8j85hh1xknfy2nqgw4ihfchr-libgcrypt-aarch64-unknown-linux-gnu-1.10.1-dev/include -I/nix/store/mqx4d57dva5y5kg91a9pnlipdc6bfcxc-libgpg-error-aarch64-unknown-linux-gnu-1.46-dev/include -g -O2 -c -o pdfgrep.o pdfgrep.cc
pdfgrep-aarch64-unknown-linux-gnu> In file included from /nix/store/cha4mfjki3pv42xnfsfrgppr3w1zmh27-aarch64-unknown-linux-gnu-stage-final-gcc-12.2.0/aarch64-unknown-linux-gnu/include/c++/12.2.0/stdlib.h:36,
pdfgrep-aarch64-unknown-linux-gnu> from pdfgrep.cc:24:
pdfgrep-aarch64-unknown-linux-gnu> /nix/store/cha4mfjki3pv42xnfsfrgppr3w1zmh27-aarch64-unknown-linux-gnu-stage-final-gcc-12.2.0/aarch64-unknown-linux-gnu/include/c++/12.2.0/cstdlib:151:11: error: 'malloc' has not been declared in '::'
pdfgrep-aarch64-unknown-linux-gnu> 151 | using ::malloc;
pdfgrep-aarch64-unknown-linux-gnu> | ^~~~~~
pdfgrep-aarch64-unknown-linux-gnu> /nix/store/cha4mfjki3pv42xnfsfrgppr3w1zmh27-aarch64-unknown-linux-gnu-stage-final-gcc-12.2.0/aarch64-unknown-linux-gnu/include/c++/12.2.0/cstdlib:164:11: error: 'realloc' has not been declared in '::'
pdfgrep-aarch64-unknown-linux-gnu> 164 | using ::realloc;
pdfgrep-aarch64-unknown-linux-gnu> | ^~~~~~~
pdfgrep-aarch64-unknown-linux-gnu> /nix/store/cha4mfjki3pv42xnfsfrgppr3w1zmh27-aarch64-unknown-linux-gnu-stage-final-gcc-12.2.0/aarch64-unknown-linux-gnu/include/c++/12.2.0/stdlib.h:65:12: error: 'malloc' has not been declared in 'std'
pdfgrep-aarch64-unknown-linux-gnu> 65 | using std::malloc;
pdfgrep-aarch64-unknown-linux-gnu> | ^~~~~~
pdfgrep-aarch64-unknown-linux-gnu> /nix/store/cha4mfjki3pv42xnfsfrgppr3w1zmh27-aarch64-unknown-linux-gnu-stage-final-gcc-12.2.0/aarch64-unknown-linux-gnu/include/c++/12.2.0/stdlib.h:73:12: error: 'realloc' has not been declared in 'std'
pdfgrep-aarch64-unknown-linux-gnu> 73 | using std::realloc;
pdfgrep-aarch64-unknown-linux-gnu> | ^~~~~~~
pdfgrep-aarch64-unknown-linux-gnu> make[2]: *** [Makefile:413: pdfgrep.o] Error 1
pdfgrep-aarch64-unknown-linux-gnu> make[2]: Leaving directory '/build/pdfgrep-2.1.2/src'
pdfgrep-aarch64-unknown-linux-gnu> make[1]: *** [Makefile:388: all-recursive] Error 1
pdfgrep-aarch64-unknown-linux-gnu> make[1]: Leaving directory '/build/pdfgrep-2.1.2'
pdfgrep-aarch64-unknown-linux-gnu> make: *** [Makefile:329: all] Error 2
```

pkgs/tools/typesetting/pdfgrep/default.nix

···
     done
   '';
 
+  configureFlags = [
+    "--with-libgcrypt-prefix=${lib.getDev libgcrypt}"
+  ];
+
   nativeBuildInputs = [ pkg-config asciidoc ];
   buildInputs = [ poppler libgcrypt pcre ];