accept the same environment variable to find GSL as in cmakes default module

CMakeModules/FindGSL.cmake

@@ -13,13 +13,13 @@
 # GSL_FOUND       - do not attempt to use if "no" or undefined.
 
 FIND_PATH (GSL_INCLUDE_DIR gsl/gsl_fft.h
-    HINTS $ENV{GSL_INCLUDE_PATH} $ENV{GSL_INCLUDE_DIR} $ENV{GSL_PREFIX}/include $ENV{GSL_DIR}/include $ENV{GSL}/include
+    HINTS $ENV{GSL_ROOT_DIR}/include $ENV{GSL_INCLUDE_PATH} $ENV{GSL_INCLUDE_DIR} $ENV{GSL_PREFIX}/include $ENV{GSL_DIR}/include $ENV{GSL}/include
     PATHS ENV CPP_INCLUDE_PATH
 )
 
 
 FIND_LIBRARY (GSL_LIBRARY gsl
-    HINTS $ENV{GSL_LIBRARY_PATH} $ENV{GSL_LIBRARY_DIR} $ENV{GSL_PREFIX}/lib $ENV{GSL_DIR}/lib $ENV{GSL}/lib
+    HINTS $ENV{GSL_ROOT_DIR}/lib $ENV{GSL_LIBRARY_PATH} $ENV{GSL_LIBRARY_DIR} $ENV{GSL_PREFIX}/lib $ENV{GSL_DIR}/lib $ENV{GSL}/lib
     PATHS ENV LIBRARY_PATH
 )
 
@@ -41,4 +41,4 @@ ELSE ()
    IF (GSL_FIND_REQUIRED)
       MESSAGE(FATAL_ERROR "Could not find GSL!")
    ENDIF ()
-ENDIF ()
+ENDIF ()
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