src/CMakeLists.txt: link libplfit against $MATH_LIBRARY

The libplfit library makes use of several math functions (exp, log,
...) , and so should be linked to libm directly when libm is required
for those functions. If it is not, then compilation may succeed
leaving the resulting library underlinked. This can lead to errors
like,

  test_discrete: symbol lookup error: .../src/libplfit.so.0: undefined
  symbol: log

when linking with lld and LDFLAGS="Wl,--as-needed". (The error above
comes from plfit's own test suite.)

Gentoo-bug: https://bugs.gentoo.org/926433

src/CMakeLists.txt

@@ -13,6 +13,7 @@ set(PLFIT_CORE_SRCS error.c gss.c kolmogorov.c lbfgs.c mt.c platform.c plfit.c o
 add_library(plfit ${PLFIT_CORE_SRCS})
 target_include_directories(plfit PUBLIC ${CMAKE_CURRENT_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR})
 set_target_properties(plfit PROPERTIES SOVERSION 0)
+target_link_libraries(plfit ${MATH_LIBRARY})
 
 if(PLFIT_USE_OPENMP AND OPENMP_FOUND)
     target_link_libraries(plfit OpenMP::OpenMP_C)