CMake: try to fix the Haiku build.

If we set CMAKE_C_FLAGS_INIT to "-fPIC" before setting the project, that
initializes CMAKE_C_FLAGS to "-fPIC" *before* prepending the contents of
the CFLAGS environment variable, and *before* doing any tests that
depend on linking, in the project() command, so it 1) adds -fPIC before
doing those tests, so they don't fail due to the object files being
unfit to link into an executable, and 2) adds the contents of CFLAGS to
CMAKE_C_FLAGS, so the user can set CFLAGS and have it affect the build.

This requires CMake 3.7 or later, so we set 3.7 as the minimum required
on Haiku.

This should fix some of #996, although more needs to be done.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6a915c32cd7e06520d5352acf17ad8031bd0e735..55d8a0e48b453593275d69bada529539d072e855 100644 (file)
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,8 +2,22 @@ if(WIN32)
     #
     # We need 3.12 or later, so that we can set policy CMP0074; see
     # below.
+    #
     cmake_minimum_required(VERSION 3.12)
-else(WIN32)
+elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Haiku")
+    #
+    # We need 3.7 or later, so we can use CMAKE_C_FLAGS_INIT; see
+    # below.
+    #
+    # We test CMAKE_HOST_SYSTEM_NAME rather than CMAKE_SYSTEM_NAME,
+    # because the former is set before project() is called (it's set
+    # from the results of uname()), but the latter isn't set until
+    # after project() returns, and we have to do those tests before
+    # calling project() so we set CMAKE_C_FLAGS_INIT before calling
+    # project().
+    #
+    cmake_minimum_required(VERSION 3.7)
+else()
     #
     # For now, require only 2.8.12, just in case somebody is
     # configuring with CMake on a "long-term support" version
@@ -17,7 +31,7 @@ else(WIN32)
     # it finds any .pc file from there.
     #
     cmake_minimum_required(VERSION 2.8.12)
-endif(WIN32)
+endif()
 
 #
 # We want find_path() and find_library() to honor {packagename}_ROOT,
@@ -84,6 +98,27 @@ set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 
 set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/Modules)
 
+#
+# See above for why we test CMAKE_HOST_SYSTEM_NAME rather than
+# CMAKE_SYSTEM_NAME.
+#
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Haiku")
+    #
+    # On Haiku, all executables are built as shared objects,
+    # and must have their code build as PIC.
+    #
+    # At least some versions of Haiku's GCC default to PIC,
+    # with a -fno-pic option for cases where that's not desired.
+    #
+    # Clang hasn't been modified in that fashion, so Clang
+    # builds of tcpdump fail.  This is Haiku bug 18258.
+    #
+    # Force the use of -fPIC (even for GCC; adding -fPIC for GCC
+    # won't break anything).
+    #
+    set(CMAKE_C_FLAGS_INIT "-fPIC")
+endif()
+
 #
 # OK, this is a royal pain.
 #
@@ -115,20 +150,6 @@ set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/Modules)
 # C++ code in tcpdump.
 #
 project(tcpdump C)
-if(CMAKE_SYSTEM_NAME STREQUAL "Haiku")
-    #
-    # Due to Haiku bug 18258, it is difficult if not impossible
-    # to get Clang to be usable as a compiler for CMake projects.
-    #
-    # If CMAKE_SIZEOF_VOID_P is undefined, we assume that means
-    # that the attempt to determine the compiler ABI failed, due
-    # the the consequences of that bug, and tell the user that
-    # this compiler doesn't work wel with CMake on Haiku
-    #
-    if(NOT CMAKE_SIZEOF_VOID_P)
-        message(FATAL_ERROR "${CMAKE_C_COMPILER} does not work well on Haiku for projects using CMake")
-    endif()
-endif()
 
 #
 # For checking if a compiler flag works and adding it if it does.