cmake: rework -Wl,--no-undefined/-Wl,-undefined,error handling

Issue tbeu#187 appears to indicate that
OpenBSD requires an explicit specification of -lc on the linker line
if -Wl,--no-undefined is used.

Unfortunately, -lc is not available on MinGW (due to the C runtime
working different on Windows), so adding it whenever -lm is also linked
against does not work.

This commit reworks the entire CMake linker flag logic:

 - Remove dependency on CMake >= 3.17.0 for some checks -- we need to
   do a manual check_c_source_compiles() anyway (due to
   check_linker_flag() not triggering the potential error when -lc is
   not supplied), so we can drop that dependency.

 - Check for -Wl,--no-undefined both without and with an explicit -lc
   on the command line. If either version works, assume we can pass
   GNU-style linker options. If only the explicit version works, also
   pass -lc explicitly. (If both work don't, because even if -lc works,
   such as on a standard Linux, one would typically not pass it
   explicitly while compiling software.)

Fixes tbeu#233

cmake/compilerOptions.cmake

@@ -88,26 +88,49 @@ check_c_source_compiles("${TEST_CODE_THOUSANDS_SEP}" HAVE_STRUCT_LCONV_THOUSANDS
 
 set(USE_GNU_LINK_FLAGS 0)
 set(USE_LLVM_MACOS_LINK_FLAGS 0)
+set(REQUIRE_EXPLICIT_LIBC_LINK 0)
 if(NOT MSVC)
-    if(${CMAKE_VERSION} VERSION_GREATER "3.17")
-        include(CheckLinkerFlag)
-        check_linker_flag(C "-Wl,--no-undefined" HAVE_LINK_NO_UNDEFINED)
-        check_linker_flag(
-            C "-Wl,--retain-symbols-file,${PROJECT_SOURCE_DIR}/src/matio.sym"
-            HAVE_LINK_RETAIN_SYMBOLS_FILE
-        )
-        check_linker_flag(C "-Wl,-undefined,error" HAVE_LINK_UNDEFINED_ERROR)
+    # OpenBSD apparently requires an explicit -lc if -Wl,--no-undefined
+    # is used, but that is NOT required on other platforms, and on even
+    # other platforms (e.g. MinGW) -lc might not even exist at all.
+    # Therefore detect if -lc works and if it is required, and omit it
+    # if it doesn't.
+    # Just running check_linker_flag() doesn't help here, because we
+    # actually need to reference a symbol from libc to cause an error.
+    # Use -shared in CMAKE_REQUIRED_LINK_OPTIONS because how symbols
+    # (esp. undefined ones) are resolved during linking can differ
+    # between shared libraries and executables.
+    set(TEST_SRC_LINK_NO_UNDEFINED "
+#include <stdlib.h>
+int main() { int* foo = (int*) malloc(sizeof(int)); free(foo); return 0; }
+")
 
-        if(HAVE_LINK_NO_UNDEFINED AND HAVE_LINK_RETAIN_SYMBOLS_FILE)
-            set(USE_GNU_LINK_FLAGS 1)
-        elseif(APPLE AND HAVE_LINK_UNDEFINED_ERROR)
-            set(USE_LLVM_MACOS_LINK_FLAGS 1)
-        endif()
-    else()
-        if(APPLE AND CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-            set(USE_LLVM_MACOS_LINK_FLAGS 1)
-        else()
-            set(USE_GNU_LINK_FLAGS 1)
-        endif()
+    set(CMAKE_REQUIRED_FLAGS "-fPIC")
+    set(CMAKE_REQUIRED_LINK_OPTIONS "-shared;-Wl,--no-undefined")
+    check_c_source_compiles("${TEST_SRC_LINK_NO_UNDEFINED}" HAVE_LINK_NO_UNDEFINED_IMPLICIT_LIBC)
+    set(CMAKE_REQUIRED_LINK_OPTIONS "-shared;-Wl,--no-undefined;-lc")
+    check_c_source_compiles("${TEST_SRC_LINK_NO_UNDEFINED}" HAVE_LINK_NO_UNDEFINED_EXPLICIT_LIBC)
+    # Reuse the same source code for other linker flag tests (This was
+    # previously guarded by CMake version >= 3.17 using
+    # check_linker_flag, but this variant should work for even older
+    # CMake versions.)
+    set(CMAKE_REQUIRED_LINK_OPTIONS "-shared;-Wl,--retain-symbols-file,${PROJECT_SOURCE_DIR}/src/matio.sym")
+    check_c_source_compiles("${TEST_SRC_LINK_NO_UNDEFINED}" HAVE_LINK_RETAIN_SYMBOLS_FILE)
+    set(CMAKE_REQUIRED_LINK_OPTIONS "-shared;-Wl,-undefined,error")
+    check_c_source_compiles("${TEST_SRC_LINK_NO_UNDEFINED}" HAVE_LINK_UNDEFINED_ERROR)
+    set(CMAKE_REQUIRED_FLAGS "")
+    set(CMAKE_REQUIRED_LINK_OPTIONS "")
+
+    if((HAVE_LINK_NO_UNDEFINED_IMPLICIT_LIBC OR HAVE_LINK_NO_UNDEFINED_EXPLICIT_LIBC) AND HAVE_LINK_RETAIN_SYMBOLS_FILE)
+        message(VERBOSE "Using GNU-style linker flags")
+        set(USE_GNU_LINK_FLAGS 1)
+    elseif(APPLE AND HAVE_LINK_UNDEFINED_ERROR)
+        message(VERBOSE "Using Apple-clang-style linker flags")
+        set(USE_LLVM_MACOS_LINK_FLAGS 1)
+    endif()
+
+    if(NOT HAVE_LINK_NO_UNDEFINED_IMPLICIT_LIBC AND HAVE_LINK_NO_UNDEFINED_EXPLICIT_LIBC)
+        message(VERBOSE "The usage of -Wl,--no-undefined requires the explicit usage of -lc on this platform.")
+        set(REQUIRE_EXPLICIT_LIBC_LINK 1)
     endif()
 endif()

cmake/src.cmake

@@ -68,7 +68,7 @@ if(STDINT_MSVC)
 endif()
 
 if(HAVE_LIBM)
-    target_link_libraries(${PROJECT_NAME} PUBLIC m c)
+    target_link_libraries(${PROJECT_NAME} PUBLIC m)
 endif()
 
 if(MSVC)
@@ -84,6 +84,10 @@ if(ZLIB_FOUND)
     target_link_libraries(${PROJECT_NAME} PUBLIC MATIO::ZLIB)
 endif()
 
+if(REQUIRE_EXPLICIT_LIBC_LINK)
+    target_link_libraries(${PROJECT_NAME} PUBLIC c)
+endif()
+
 set_target_properties(${PROJECT_NAME} PROPERTIES POSITION_INDEPENDENT_CODE ${MATIO_PIC})
 # Convert matio_LIB_VERSIONINFO libtool version format into SOVERSION
 # Convert from ":" separated into CMake list format using ";"