guix: Adjust for Qt 6

1. Do not set `C{PLUS}_INCLUDE_PATH` variables

The build system for Qt 6 differs entirely from that of Qt 5. Building a
set of native Qt 6 tools now forms a separate step when cross-compiling.
Under these new circumstances, the `C{PLUS}_INCLUDE_PATH` environment
variables may alter the default include directories for both native and
cross compilers.

Previously, we explicitly unset these variables when invoking clang for
cross-compiling; however, that approach proved suboptimal (see #30451).

This change sets the native toolchain for dependencies explicitly,
rather than relying on the `C{PLUS}_INCLUDE_PATH` environment variables.
Additionally, it facilitates the transition towards using clang for
building native tools when cross-compiling for macOS.

2. Add `ninja` package.

3. Adjust allowed symbol lists.

contrib/guix/libexec/build.sh

@@ -69,8 +69,9 @@ unset CPLUS_INCLUDE_PATH
 unset OBJC_INCLUDE_PATH
 unset OBJCPLUS_INCLUDE_PATH
 
-export C_INCLUDE_PATH="${NATIVE_GCC}/include"
-export CPLUS_INCLUDE_PATH="${NATIVE_GCC}/include/c++:${NATIVE_GCC}/include"
+# Set native toolchain
+build_CC="${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" ;; # Required for qt/qmake
@@ -171,6 +172,8 @@ make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${SOURCES_PATH+SOURCES_PATH="$SOURCES_PATH"} \
                                    ${BASE_CACHE+BASE_CACHE="$BASE_CACHE"} \
                                    ${SDK_PATH+SDK_PATH="$SDK_PATH"} \
+                                   ${build_CC+build_CC="$build_CC"} \
+                                   ${build_CXX+build_CXX="$build_CXX"} \
                                    x86_64_linux_CC=x86_64-linux-gnu-gcc \
                                    x86_64_linux_CXX=x86_64-linux-gnu-g++ \
                                    x86_64_linux_AR=x86_64-linux-gnu-gcc-ar \
@@ -181,8 +184,6 @@ make -C depends --jobs="$JOBS" HOST="$HOST" \
 case "$HOST" in
     *darwin*)
         # Unset now that Qt is built
-        unset C_INCLUDE_PATH
-        unset CPLUS_INCLUDE_PATH
         unset LIBRARY_PATH
         ;;
 esac