Compiler errors for some of the nominated installer mechanisms

[dwinsemius]

Recent difficulty in installing via some some but not all of the package copies:
These successes and failures on:
R version 4.3.1 (2023-06-16)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.6 LTS
But my investigation was pursuant to a SO Q that reported on a Windoze bo and who also had success with the same method..

Success with:
remotes::install_github("hrbrmstr/mgrs")

Failure with:
remotes::install_git("https://git.rud.is/hrbrmstr/mgrs.git")
...
curity -Wdate-time -D_FORTIFY_SOURCE=2 -Wno-ignored-attributes -c main.cpp -o main.o
main.cpp: In function ‘Rcpp::DataFrame mgrs_to_latlng(std::vector<std::__cxx11::basic_string >, bool, bool)’:
main.cpp:41:42: error: ‘PI’ was not declared in this scope
lat_vec[i] = degrees ? lat * 180.0/PI : lat;

And failure with:
install.packages("mgrs", repos = "https://cinc.rud.is")
...
curity -Wdate-time -D_FORTIFY_SOURCE=2 -Wno-ignored-attributes -c main.cpp -o main.o
main.cpp: In function ‘Rcpp::DataFrame mgrs_to_latlng(std::vector<std::__cxx11::basic_string >, bool, bool)’:
main.cpp:41:42: error: ‘PI’ was not declared in this scope
lat_vec[i] = degrees ? lat * 180.0/PI : lat;