build: don't use undocumented -export-dynamic option

GNU libtool is documented to support -export-dynamic and GNU ld is documented to support --export-dynamic. However, GCC isn't documented to support them. While GCC does support -export-dynamic for backwards compatibility, Clang doesn't support it and interprets it wrongly. This causes warnings to be shown during linking because Clang splits it into '-e xport-dynamic' and the linker can't find 'xport-dynamic' symbol.

This fixes the warning when compiling with Clang.

[30/56] Linking target src/gnome-builder.
/usr/bin/ld: warning: cannot find entry symbol xport-dynamic; defaulting to 0x2E5000

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