Remove GNU-specific flags from Linux.cmake

We remove the shared library compile/link flags "-fPIC" and "-shared"
because they are not provided by all compilers on Linux.  This allows us
to drop code from the Linux-XL-*.cmake files that erases the bad flags.
All other supported compilers already provide their correct flags for
Linux in their own platform information files.

Modules/Platform/Linux-XL-C.cmake

@@ -1,2 +1 @@
-SET(CMAKE_SHARED_LIBRARY_C_FLAGS "")
 SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-qmkshrobj")

Modules/Platform/Linux-XL-CXX.cmake

@@ -1,3 +1 @@
-SET(CMAKE_SHARED_LIBRARY_C_FLAGS "")
-SET(CMAKE_SHARED_LIBRARY_CXX_FLAGS "")
 SET(CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS "-qmkshrobj")

Modules/Platform/Linux-XL-Fortran.cmake

@@ -1,2 +1 @@
-SET(CMAKE_SHARED_LIBRARY_C_FLAGS "")
 SET(CMAKE_SHARED_LIBRARY_CREATE_Fortran_FLAGS "-qmkshrobj")

Modules/Platform/Linux.cmake

@@ -1,7 +1,4 @@
-# GCC is the default compiler on Linux.
 SET(CMAKE_DL_LIBS "dl")
-SET(CMAKE_SHARED_LIBRARY_C_FLAGS "-fPIC")        
-SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared")
 SET(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG "-Wl,-rpath,")
 SET(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG_SEP ":")
 SET(CMAKE_SHARED_LIBRARY_RPATH_LINK_C_FLAG "-Wl,-rpath-link,")