Do not overwrite default linker flags so that LDFLAGS environment variable is considered

[traversaro]

If a CMake build system of a projects wants to specify additional linked flags, those should be appended to the CMAKE_EXE_LINKER_FLAGS CMake variable. However, the original value of CMAKE_EXE_LINKER_FLAGS should not been overwritten, as this would lead to the LDFLAGS environment variable to be ignored, leading to linking errors in some environments that exploit this environment variable to specify the location of the libraries.

For more details see:

• Do not overwrite default linker flags so that LDFLAGS environment variable is considered gazebosim/gazebo-classic#2922
• Conda Linux CI build fail due to linking problem in Wearables robotology-superbuild#681 (comment)

[traversaro]

This PR fixes a bug in any case, but the main reason why I opened it is that I hope that it will fix robotology/robotology-superbuild#848 .

[Nicogene]

Merged, thanks!