Update GROMACS: GPU option as tag

MPI/gromacs/2020/variants

-gromacs/2020_sp  unstable gcc/7.4.0 openmpi/3.1.5_merlin6 cuda/10.0.130 Python/3.7.4 b:cmake/3.14.0
-gromacs/2020.2_sp  unstable gcc/7.5.0 openmpi/3.1.6_slurm cuda/10.0.130 Python/3.7.4 b:cmake/3.15.5
+gromacs/2020_sp        unstable gcc/7.4.0 openmpi/3.1.5_merlin6 cuda/10.0.130 Python/3.7.4 b:cmake/3.14.0
+gromacs/2020.2_sp      unstable gcc/7.5.0 openmpi/3.1.6_slurm cuda/10.0.130 Python/3.7.4 b:cmake/3.15.5
+gromacs/2020.2_sp_gpu  unstable gcc/7.5.0 openmpi/4.0.4_slurm cuda/10.0.130 Python/3.7.4 b:cmake/3.15.5

MPI/gromacs/build

@@ -11,10 +11,6 @@ else
 fi
 
 pbuild::pre_configure() {
-  # For enabling GPU, set to 'yes'
-  local enable_gpu='yes'
-
-  echo here
   pbuild::add_configure_args "-DCMAKE_BUILD_TYPE=RELEASE"
   pbuild::add_configure_args "-DGMX_MPI=on"
 
@@ -24,11 +20,12 @@ pbuild::pre_configure() {
     pbuild::add_configure_args "-DGFMX_DOUBLE=off"
   fi
 
-  if [[ "${enable_gpu}" == 'yes' ]]; then
+  if pbuild::use_flag "gpu"; then
  	pbuild::add_configure_args "-DGMX_GPU=on"
   else
  	pbuild::add_configure_args "-DGMX_GPU=off"
   fi
+
   # For GMX_SIMD the lowest instruction number you think might work on all nodes
   #   |- Available numbers: http://manual.gromacs.org/documentation/2020/install-guide/index.html#simd-support
   #   |- AVX_512 is only for newest Intel processors (Xeon Scalable Gold, Core i9, etc.)