gmp provides pkg-config files gmp.pc and gmpxx.pc since version 6.2.0. Perhaps STklos could switch to using those. This would eliminate the current need for the user to manually specify CPPFLAGS and LDFLAGS for locating gmp. (Or, if STklos still wishes to support gmp versions earlier than 6.2.0, it could try pkg-config first and fall back to the old method if needed.)
The configure script finds most dependencies with pkg-config, but not gmp:
https://github.com/egallesio/STklos/blob/393b8f4ecf4117c8c23e3c32ce3caa050218fb03/configure.ac#L284-L293
gmp provides pkg-config files gmp.pc and gmpxx.pc since version 6.2.0. Perhaps STklos could switch to using those. This would eliminate the current need for the user to manually specify CPPFLAGS and LDFLAGS for locating gmp. (Or, if STklos still wishes to support gmp versions earlier than 6.2.0, it could try pkg-config first and fall back to the old method if needed.)