Add run-time interface for build-time configuration

[jedbrown]

This provides the interfaces requested in #1732

[jrwrigh]

IDK if it's relevant for this feature, but if I try and run git describe --dirty on libCEED main, then I get an error:

fatal: No names found, cannot describe anything.

I didn't get this error when I ran it on a branch initially, but now it only returns the error after pulling main.

Edit:
pulling this branch gets the same thing.

$ make print-GIT_DESCRIBE
[ variable name]: GIT_DESCRIBE
[        origin]: file
[        flavor]: recursive
[         value]: $(shell git describe --dirty || printf "unknown\n")
fatal: No names found, cannot describe anything.
[expanded value]: unknown

This is with git version 2.49.0 on Arch. Not sure if that makes a difference.

Edit 2:
Resolved above: #1792 (comment)

[jeremylt]

Issue was that old versions of make do not have file as a command.

With these changes:

$ ./build/t010-config /cpu/self
Git: v0.12.0-969-gd7fa60a8-dirty
CC = gcc
CXX = g++
FC = gfortran
NVCC = /bin/nvcc
NVCC_CXX = g++
HIPCC = /bin/hipcc
OPT = -O -march=native -g -ffp-contract=fast -fopenmp-simd
CFLAGS = -O -march=native -g -ffp-contract=fast -fopenmp-simd -fPIC -std=c99 -Wall -Wextra -Wno-unused-parameter -MMD -MP  
CPPFLAGS = -I./include
CXXFLAGS = -O -march=native -g -ffp-contract=fast -fopenmp-simd -fPIC -std=c++11 -Wall -Wextra -Wno-unused-parameter -MMD -MP 
FFLAGS = -O -march=native -g -ffp-contract=fast -fopenmp-simd -fPIC -cpp -Wall -Wextra -Wno-unused-parameter -Wno-unused-dummy-argument -MMD -MP 
NVCCFLAGS = -ccbin g++ -Xcompiler '-O -march=native -g -ffp-contract=fast -fopenmp-simd' -Xcompiler -fPIC
HIPCCFLAGS = -O -march=native -g -ffp-contract=fast -fPIC -munsafe-fp-atomics
SYCLFLAGS =  -fPIC -std=c++17 -O -march=native -g -ffp-contract=fast -fopenmp-simd -fPIC -Wall -Wextra -Wno-unused-parameter -MMD -MP -O -march=native -g -ffp-contract=fast
AR = ar
ARFLAGS = crD
LDFLAGS = -Wl,-rpath,/home/jeremy/Dev/libCEED/lib -Llib
LDLIBS = 
LIBCXX = -lstdc++
SED = sed
MAGMA_DIR = ../magma
OCCA_DIR = ../occa/install
XSMM_DIR = ../libxsmm
CUDA_DIR = 
CUDA_ARCH = 
MFEM_DIR = ../mfem
PETSC_DIR = 
NEK5K_DIR = /home/jeremy/Dev/Nek5000
ROCM_DIR = 
HIP_ARCH = 
SYCL_DIR =