Allow CMake program to be specified with env. var.

If you are using a custom CMake, you should be able to set the
environment variable "CMAKE" and have the configure script pick up
the default from there so that you don't have to specify the option
every time.

configure

@@ -53,6 +53,7 @@ Some influential environment variables:
   FFLAGS      Fortran 77 compiler flags
   FC          Fortran compiler command
   FCFLAGS     Fortran compiler flags
+  CMAKE_COMMAND  Executable used to run cmake scripts
 
 Use these variables to override the choices made by \`configure' or to help
 it to find libraries and programs with nonstandard names/locations.
@@ -90,10 +91,12 @@ option_check=yes
 FEATURES=
 
 # command that launches cmake; look for 2.8 if available
-if which cmake28 >/dev/null 2>&1; then
-CMAKE=cmake28
-else
-CMAKE=cmake
+if [ "${CMAKE_COMMAND}" = "" ]; then
+  if which cmake28 >/dev/null 2>&1; then
+    CMAKE_COMMAND=cmake28
+  else
+    CMAKE_COMMAND=cmake
+  fi
 fi
 
 # long arguments are implemented by putting a dash character followed by
@@ -121,7 +124,7 @@ while getopts -- ":-:" optchar; do
           # the parameter to this option is an executable program, so
           # skip the directory test in that case
           if [ "${pkgname}" = "cmake" ]; then
-            CMAKE="${pkgloc}"
+            CMAKE_COMMAND="${pkgloc}"
             break
           fi
           # expand to full path since CMake changes to source directory (!)
@@ -268,7 +271,7 @@ shift $((OPTIND-1))
 ENVVARS=${@/ACLOCAL_*=*/}
 
 # pass everything on to CMake
-CMDLINE="env ${ENVVARS} ${CMAKE} \"$(dirname "$0")\" \"-DCMAKE_INSTALL_PREFIX=$prefix\" -DCMAKE_BUILD_TYPE=${buildtype}${pch_use}${silent_rules}${debug_loc} ${FEATURES}"
+CMDLINE="env ${ENVVARS} ${CMAKE_COMMAND} \"$(dirname "$0")\" \"-DCMAKE_INSTALL_PREFIX=$prefix\" -DCMAKE_BUILD_TYPE=${buildtype}${pch_use}${silent_rules}${debug_loc} ${FEATURES}"
 echo --- calling CMake for opm-core ---
 echo ${CMDLINE}
 eval exec ${CMDLINE}