Use CheckSymbolExists to verify existence of lgamma_r

Using CheckSourceCompiles doesn't actually test linking.
CheckSymbolExists does. On MacOS, it seems that lgamma_r needs
_REENTRANT macro to defined to the thread-safe version of lgamma to be
visible, so we make the check by making use of
CMAKE_REQUIRED_DEFINITIONS.

core/CMakeLists.txt

@@ -1,4 +1,4 @@
-include(CheckCXXSourceCompiles)
+include(CheckSymbolExists)
 
 file(GLOB_RECURSE CORE_SRCS *.cpp *.h)
 
@@ -24,11 +24,10 @@ add_library(mrtrix-core SHARED ${CORE_SRCS})
 add_library(mrtrix::core ALIAS mrtrix-core)
 
 # Check to see if we can use lgamma_r() function in custom Math::betaincreg()
-check_cxx_source_compiles("
-  extern \"C\" double lgamma_r(double, int *);
-  int main() { int sign; lgamma_r(1.0, &sign); }"
-  MRTRIX_HAVE_LGAMMA_R
-)
+# The function is defined under _REENTRANT on some systems (e.g. MacOS)
+list(APPEND CMAKE_REQUIRED_DEFINITIONS -D_REENTRANT)
+check_symbol_exists(lgamma_r "math.h" MRTRIX_HAVE_LGAMMA_R)
+list(REMOVE_AT CMAKE_REQUIRED_DEFINITIONS -1)
 
 if(MRTRIX_HAVE_LGAMMA_R)
     message(STATUS "Found lgamma_r() function")
@@ -71,6 +70,10 @@ target_compile_definitions(mrtrix-core PUBLIC
     MRTRIX_HAVE_LGAMMA_R=$<BOOL:${MRTRIX_HAVE_LGAMMA_R}>
 )
 
+if(APPLE AND MRTRIX_HAVE_LGAMMA_R)
+    target_compile_definitions(mrtrix-core PRIVATE _REENTRANT=1)
+endif()
+
 if(PNG_FOUND)
     target_compile_definitions(mrtrix-core PUBLIC MRTRIX_PNG_SUPPORT)
     target_link_libraries(mrtrix-core PUBLIC PNG::PNG)