Newer PROJ version causes build error; API has changed recently

[guspolly]

I'm attempting to build redistricter on Debian testing, which currently uses PROJ version 7.2.1.

The first notice that something was amiss re: PROJ was this compile error:

In file included from /mnt/e/redistricter/cpp/linkfixup.cpp:11:
/usr/include/proj_api.h:37:2: error: #error 'To use the proj_api.h you must define the macro ACCEPT_USE_OF_DEPRECATED_PROJ_API_H'
   37 | #error 'To use the proj_api.h you must define the macro ACCEPT_USE_OF_DEPRECATED_PROJ_API_H'
      |  ^~~~~
make: *** [<builtin>: /mnt/e/redistricter/cpp/linkfixup.o] Error 1

I figured out how and where to do that (I'm not a coder), and added #define ACCEPT_USE_OF_DEPRECATED_PROJ_API_H 1 before #include <proj_api.h> in linkfixup.cpp.

Then make gave these errors (line numbers are +1 compared to git because I added the macro at line 11):

/usr/bin/ld: /mnt/e/redistricter/cpp/linkfixup.o: in function `Proj::initRadians(double, double)':
/mnt/e/redistricter/cpp/linkfixup.cpp:58: undefined reference to `pj_init_plus'
/usr/bin/ld: /mnt/e/redistricter/cpp/linkfixup.cpp:59: undefined reference to `pj_init_plus'
/usr/bin/ld: /mnt/e/redistricter/cpp/linkfixup.o: in function `Proj::clear()':
/mnt/e/redistricter/cpp/linkfixup.cpp:71: undefined reference to `pj_free'
/usr/bin/ld: /mnt/e/redistricter/cpp/linkfixup.cpp:75: undefined reference to `pj_free'
/usr/bin/ld: /mnt/e/redistricter/cpp/linkfixup.o: in function `Proj::transform(double*, double*)':
/mnt/e/redistricter/cpp/linkfixup.cpp:81: undefined reference to `pj_transform'
collect2: error: ld returned 1 exit status
make: *** [cpp/cpp.make:51: linkfixup] Error 1

According to https://proj.org/development/migration.html, the PROJ API was overhauled in version 6, and the functions pj_init_plus, pj_free, and pj_transform are among those that were changed.