-Fix R_NO_REMAP warning: "R_NO_REMAP" redefined compilation warning

[tylermorganwall]

Compiling RcppExports.cpp when linking RcppThread can lead to a warning about redefining R_NO_REMAP: this PR adds a preprocessor guard to ensure it's only defined once.

See the following build logs (below is relevant excerpt):

2024-12-29T17:10:15.1806052Z g++  -std=gnu++20 -I"C:/R/include" -DNDEBUG  -I'D:/a/_temp/Library/Rcpp/include' -I'D:/a/_temp/Library/RcppThread/include' -I'D:/a/_temp/Library/progress/include' -I'D:/a/_temp/Library/spacefillr/include' -I'D:/a/_temp/Library/testthat/include'   -I"C:/rtools44/x86_64-w64-mingw32.static.posix/include"  -DRAY_REPRODUCE_PERLIN -DSTRICT_R_HEADERS -DRAY_WINDOWS -DHAS_SSE -DRAYSIMD -DRAYSIMDVECOFF -DR_NO_REMAP   -O2 -Wall  -mfpmath=sse -msse2 -mstackrealign  -c RcppExports.cpp -o RcppExports.o
2024-12-29T17:10:15.2190025Z In file included from D:/a/_temp/Library/RcppThread/include/RcppThread.h:9,
2024-12-29T17:10:15.2190688Z                  from RcppExports.cpp:4:
2024-12-29T17:10:15.2191504Z D:/a/_temp/Library/RcppThread/include/RcppThread/RMonitor.hpp:10: warning: "R_NO_REMAP" redefined
2024-12-29T17:10:15.2192987Z    10 | #define R_NO_REMAP
2024-12-29T17:10:15.2193273Z       | 
2024-12-29T17:10:15.2193735Z <command-line>: note: this is the location of the previous definition

[tnagler]

Thanks!