guix: fix macOS build

These lines were present in the original code, but a comment said:
"Required for qt/qmake"

Well, not just for qt it seems. Fixes #5.

Author: Sjors

contrib/guix/libexec/build.sh

@@ -74,6 +74,7 @@ build_CC="${NATIVE_GCC}/bin/gcc -isystem ${NATIVE_GCC}/include"
 build_CXX="${NATIVE_GCC}/bin/g++ -isystem ${NATIVE_GCC}/include/c++ -isystem ${NATIVE_GCC}/include"
 
 case "$HOST" in
+    *darwin*) export LIBRARY_PATH="${NATIVE_GCC}/lib" ;;
     *mingw*) export LIBRARY_PATH="${NATIVE_GCC}/lib" ;;
     *)
         NATIVE_GCC_STATIC="$(store_path gcc-toolchain static)"
@@ -179,6 +180,12 @@ make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    x86_64_linux_NM=x86_64-linux-gnu-gcc-nm \
                                    x86_64_linux_STRIP=x86_64-linux-gnu-strip
 
+case "$HOST" in
+    *darwin*)
+        # Unset now that Qt is built
+        unset LIBRARY_PATH
+        ;;
+esac
 
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