Search libclang in LIBRARY_PATH too.

[jeandudey]

Systems like Guix (and maybe Nix) set the LIBRARY_PATH variable for compilers to find libraries when in an environment, for example:

guix shell --pure gcc-toolchain clang --search-paths
The following derivation will be built:
  /gnu/store/vh5jffs835hcml8pg9wlsbdnygfmfwq8-profile.drv

building CA certificate bundle...
listing Emacs sub-directories...
building fonts directory...
building directory of Info manuals...
building profile with 2 packages...
export PATH="/gnu/store/akpz799zak3qzh73m6nbql5i3mljnny5-profile/bin:/gnu/store/akpz799zak3qzh73m6nbql5i3mljnny5-profile/sbin"
export C_INCLUDE_PATH="/gnu/store/akpz799zak3qzh73m6nbql5i3mljnny5-profile/include"
export CPLUS_INCLUDE_PATH="/gnu/store/akpz799zak3qzh73m6nbql5i3mljnny5-profile/include/c++:/gnu/store/akpz799zak3qzh73m6nbql5i3mljnny5-profile/include"
export OBJC_INCLUDE_PATH="/gnu/store/akpz799zak3qzh73m6nbql5i3mljnny5-profile/include"
export LIBRARY_PATH="/gnu/store/akpz799zak3qzh73m6nbql5i3mljnny5-profile/lib"

This variable is used by GCC when linking libraries, e.g. when passing -lclang without the need to pass the path to the library in the arguments.