Added JULIA_LDFLAGS and JULIA_LIBS to Makefile.in

[sebasguts]

This is a follow-up to gap-system/gap#3248

[codecov]

Codecov Report

Merging #189 into master will decrease coverage by 0.39%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #189     +/-   ##
=========================================
- Coverage   72.77%   72.38%   -0.4%     
=========================================
  Files          43       43             
  Lines        2336     2303     -33     
=========================================
- Hits         1700     1667     -33     
  Misses        636      636

Impacted Files	Coverage Δ
LibGAP.jl/src/initialization.jl	90% <0%> (-2.6%)	⬇️
JuliaInterface/src/calls.c	99.13% <0%> (-0.06%)	⬇️
JuliaInterface/src/calls.h	100% <0%> (ø)	⬆️
JuliaInterface/src/convert.c	97.14% <0%> (+2.69%)	⬆️

[fingolfin]

Fine by me. Of course ideally, we should also check if those JULIA_* variables are actually defined, just in case somebody is pointing us at an older GAP version.

[sebasguts]

Good idea.