[xlqc][omp] fix: discover GSL paths via gsl-config

[mhalk]

GSL_INC and GSL_LIB were hardcoded placeholders ("/path/to/gsl/...").
Use gsl-config to discover the installed GSL at build time.

Assisted-by: Claude Opus 4.6

[zjin-lcf]

With no gsl-config on PATH, both variables expand to empty strings and
2>/dev/null hides the "command not found":

GSL_INC=[]
GSL_LIB=[]
LDFLAGS=[]

xlqc-omp/main.cpp includes <gsl/gsl_math.h> and friends, and the -lgsl -lgslcblas flags disappear along with the paths, so the user now sees a missing-header or undefined-reference error with no hint that GSL is the problem. The old placeholder at least printed /path/to/gsl/include in the error. A fallback would keep both properties, for example assigning with ?= and defaulting to -lgsl -lgslcblas when gsl-config isn't found, or dropping 2>/dev/null so the shell error surfaces.

The stated precedent doesn't hold up. The PR body says this matches "how e.g. mriQ-omp resolves external libraries," but mriQ-omp/Makefile has no external library at all: its LDFLAGS is empty and its only include is -I../mriQ-cuda. No Makefile in the repo currently uses gsl-config; the closest existing pattern is $(shell $(MPICC) --showme:compile) in lulesh-cuda/LULESH/cuda/build/Makefile. Worth correcting the description so reviewers aren't looking for a precedent that isn't there.

[mhalk]

Thanks for the feedback, the variables can now be overridden.
Please let me know if this is what you had in mind.