qmake: Add QMAKE_SONAME_PREFIX variable

If defined, the value of this variable is prepended to the built shared
library's SONAME identifier.

For more information, see: qmake/doc/src/qmake-manual.qdoc#qmake-soname-prefix

Task-number: QTBUG-31814
Change-Id: I4bceaf0c93162e4fad6bb424af1b82e74d38acdc
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>

qmake/doc/snippets/code/doc_src_qmake-manual.pro

@@ -970,3 +970,15 @@ int main() { return featureFunction(); }
 # <project root>/project.pro
 qtCompileTest(test)
 #! [182]
+
+#! [183]
+# <project root>/project.pro
+QMAKE_SONAME_PREFIX = @rpath
+#! [183]
+
+#! [184]
+# <project root>/project.pro
+QMAKE_SONAME_PREFIX = @executable_path/../Frameworks
+QMAKE_SONAME_PREFIX = @loader_path/Frameworks
+QMAKE_SONAME_PREFIX = /Library/Frameworks
+#! [184]

qmake/doc/src/qmake-manual.qdoc

@@ -2071,6 +2071,39 @@
     qmake or \l{#QMAKESPEC}{qmake.conf} and rarely
     needs to be modified.
 
+    \section1 QMAKE_SONAME_PREFIX
+
+    If defined, the value of this variable is used as a path to be prepended to
+    the built shared library's \c SONAME identifier. The \c SONAME is the
+    identifier that the dynamic linker will later use to reference the library.
+    In general this reference may be a library name or full library path. On OS
+    X and iOS, the path may be specified relatively using the following
+    placeholders:
+
+    \table
+    \header \li Placeholder  \li Effect
+    \row \li @rpath
+         \li Expands to paths defined by LC_RPATH mach-o commands in
+             the current process executable or the referring libraries.
+    \row \li @executable_path
+         \li Expands to the current process executable location.
+    \row \li @loader_path
+         \li Expands to the referring executable or library location.
+    \endtable
+
+    In most cases, using \c @rpath is sufficient and recommended:
+
+    \snippet code/doc_src_qmake-manual.pro 183
+
+    However, the prefix may be also specified using different placeholders, or
+    an absolute path, such as one of the following:
+
+    \snippet code/doc_src_qmake-manual.pro 184
+
+    For more information, see
+    \l{https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/dyld.1.html}{dyld}
+    documentation on dynamic library install names.
+
     \section1 QMAKE_TARGET
 
     Specifies the name of the project target.  The value of this

qmake/generators/unix/unixmake2.cpp

@@ -1265,6 +1265,13 @@ void UnixMakefileGenerator::init2()
                     if(!instpath.endsWith(Option::dir_sep))
                         instpath += Option::dir_sep;
                     soname.prepend(instpath);
+                } else if (!project->isEmpty("QMAKE_SONAME_PREFIX")) {
+                    QString sonameprefix = project->first("QMAKE_SONAME_PREFIX").toQString();
+                    if (!sonameprefix.startsWith('@') && !sonameprefix.startsWith('$'))
+                        sonameprefix = Option::fixPathToTargetOS(sonameprefix, false);
+                    if (!sonameprefix.endsWith(Option::dir_sep))
+                        sonameprefix += Option::dir_sep;
+                    soname.prepend(sonameprefix);
                 }
                 project->values("QMAKE_LFLAGS_SONAME").first() += escapeFilePath(soname);
             }