FindGSL: Fix gl_version.h extraction regex

Fix the regex to match versions with only two digits, such as `2.3`.

Modules/FindGSL.cmake

@@ -135,7 +135,7 @@ if( NOT GSL_VERSION )
   # 2. If gsl-config is not available, try looking in gsl/gsl_version.h
   if( NOT GSL_VERSION AND EXISTS "${GSL_INCLUDE_DIRS}/gsl/gsl_version.h" )
     file( STRINGS "${GSL_INCLUDE_DIRS}/gsl/gsl_version.h" gsl_version_h_contents REGEX "define GSL_VERSION" )
-    string( REGEX REPLACE ".*([0-9].[0-9][0-9]).*" "\\1" GSL_VERSION ${gsl_version_h_contents} )
+    string( REGEX REPLACE ".*([0-9]\\.[0-9][0-9]?).*" "\\1" GSL_VERSION ${gsl_version_h_contents} )
   endif()
 
   # might also try scraping the directory name for a regex match "gsl-X.X"