Set ENABLE_EXPORTS property for rtl_airband

[aaron-sierra]

Make sure that exported functions (i.e. {device}_input_new()) will land in the dynamic symbol table, regardless of level of optimization.

I experienced missing symbols using GCC (10.3.1) with Buildroot (2022-11-rc2). That build environment doesn't leverage the "historical" implicit exporting enabled on other distros through the following module:

/usr/share/cmake-{cmake-version}/Modules/Platform/Linux-GNU.cmake

---

This is an improved implementation superseding #338

[aaron-sierra]

Before

Note -Wl,--export-dynamic not in linking command.

[100%] Linking CXX executable rtl_airband
cd /home/user/src/buildroot/output/build/rtlsdr-airband-v4.0.2/src && /home/user/src/buildroot/output/host/bin/cmake -E cmake_link_script CMakeFiles/rtl_airband.dir/link.txt --verbose=1
/home/user/src/buildroot/output/host/bin/aarch64-none-linux-gnu-g++ --sysroot=/home/user/src/buildroot/output/host/aarch64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -D_FORTIFY_SOURCE=1 -Wall -Wextra  -DNDEBUG CMakeFiles/rtl_airband_base.dir/config.cpp.o CMakeFiles/rtl_airband_base.dir/input-common.cpp.o CMakeFiles/rtl_airband_base.dir/input-file.cpp.o CMakeFiles/rtl_airband_base.dir/input-helpers.cpp.o CMakeFiles/rtl_airband_base.dir/mixer.cpp.o CMakeFiles/rtl_airband_base.dir/output.cpp.o CMakeFiles/rtl_airband_base.dir/rtl_airband.cpp.o CMakeFiles/rtl_airband_base.dir/squelch.cpp.o CMakeFiles/rtl_airband_base.dir/util.cpp.o CMakeFiles/rtl_airband_base.dir/udp_stream.cpp.o CMakeFiles/rtl_airband_base.dir/version.cpp.o CMakeFiles/rtl_airband_base.dir/input-rtlsdr.cpp.o -o rtl_airband  -ldl -lm -lpthread -lconfig++ -lmp3lame -lshout -lrtlsdr -lfftw3f

Input config functions are not in dynamic symbol table.

$ readelf --dyn-syms buildroot/output/target/usr/bin/rtl_airband | grep _input_new
$ echo $?
1

After

Note -Wl,--export-dynamic in linking command.

[100%] Linking CXX executable rtl_airband
cd /home/user/src/buildroot/output/build/rtlsdr-airband-bd86f1afccc1a19ff9d22022a1aac4a9b6bbfd61/src && /home/user/src/buildroot/output/host/bin/cmake -E cmake_link_script CMakeFiles/rtl_airband.dir/link.txt --verbose=1
/home/user/src/buildroot/output/host/bin/aarch64-none-linux-gnu-g++ --sysroot=/home/user/src/buildroot/output/host/aarch64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -D_FORTIFY_SOURCE=1 -Wall -Wextra  -DNDEBUG -Wl,--export-dynamic CMakeFiles/rtl_airband_base.dir/config.cpp.o CMakeFiles/rtl_airband_base.dir/input-common.cpp.o CMakeFiles/rtl_airband_base.dir/input-file.cpp.o CMakeFiles/rtl_airband_base.dir/input-helpers.cpp.o CMakeFiles/rtl_airband_base.dir/mixer.cpp.o CMakeFiles/rtl_airband_base.dir/output.cpp.o CMakeFiles/rtl_airband_base.dir/rtl_airband.cpp.o CMakeFiles/rtl_airband_base.dir/squelch.cpp.o CMakeFiles/rtl_airband_base.dir/util.cpp.o CMakeFiles/rtl_airband_base.dir/udp_stream.cpp.o CMakeFiles/rtl_airband_base.dir/version.cpp.o CMakeFiles/rtl_airband_base.dir/input-rtlsdr.cpp.o -o rtl_airband  -ldl -lm -lpthread -lconfig++ -lmp3lame -lshout -lrtlsdr -lfftw3f

Input config functions are in dynamic symbol table.

$ readelf --dyn-syms buildroot/output/target/usr/bin/rtl_airband | grep _input_new
   320: 000000000000c444   196 FUNC    GLOBAL DEFAULT   13 file_input_new
   334: 0000000000011bdc   216 FUNC    GLOBAL DEFAULT   13 rtlsdr_input_new
$ echo $?
0

[szpajder]

Merged, thanks.