Set -fno-PIE by default as a Ubuntu/Mint workaround

Older versions of Ubuntu and its derivates have an issue where the
`file` utility will interpret Position Independent Executables as
shared libraries:

https://bugs.launchpad.net/ubuntu/+source/file/+bug/1747711

As a workaround, allow setting the option COMPILE_WITH_FPIE
(defaults to OFF) and add a suitable message for maintainers in both
the CMake code, during configuration runs and in ccmake.

Update the build instructions in the README to reflect this change.

Closes vegastrike#94

README.md

@@ -251,7 +251,7 @@ sudo apt-get -y install git cmake python-dev build-essential automake autoconf l
    ccmake ../engine
    # (configure/edit options to taste in ccmake, press 'c' to save the selected options
    # and press 'g' to update the build configuration files used by the make build tool)
-   make -jN # (where N is the number of available CPU threads/cores on your system)
+   cmake --build . -jN # (where N is the number of available CPU threads/cores on your system)
    mkdir ../bin && cp vegastrike ../bin/ && cp setup/vssetup ../bin/ && cd ..
    ```
 
@@ -260,13 +260,24 @@ sudo apt-get -y install git cmake python-dev build-essential automake autoconf l
    ```bash
    mkdir build & cd build
    cmake ../engine
-   make -jN # (where N is the number of available CPU threads/cores on your system)
+   cmake --build . -jN # (where N is the number of available CPU threads/cores on your system)
    mkdir ../bin && cp vegastrike ../bin/ && cp setup/vssetup ../bin/ && cd ..
    ```
+
+   *TIPS:*
+
    To enable/disable compile-time options with cmake, use `cmake -D<option>`. Example:
 
    ```bash
-   cmake ../engine -DUSE_PYTHON_3=ON -DCPU_SMP=2 -DCPUINTEL_native=ON -CMAKE_BUILD_TYPE=Debug
+   cmake ../engine -COMPILE_WITH_FPIE=ON -DUSE_PYTHON_3=ON -DCPU_SMP=2 -DCPUINTEL_native=ON -CMAKE_BUILD_TYPE=Debug
+   ```
+   In the above example, a non-Ubuntu/Mint packager has wisely decided to compile a
+   Position Independent Executable because his OS isn't
+   [buggy](https://bugs.launchpad.net/ubuntu/+source/file/+bug/1747711) when doing so.
+
+   To enable verbose output for debugging purposes (will show compilation commands), supply the `-v` flag:
+   ```bash
+   cmake --build . -v
    ```
 
 [Link to list of dependencies in wiki](http://vegastrike.sourceforge.net/wiki/HowTo:Compile_from_CVS)

engine/CMakeLists.txt

@@ -521,7 +521,6 @@ if(CMAKE_COMPILER_IS_GNUCXX)
     add_definitions(-std=gnu++03)
 endif()
 
-
 # Provide boolean options for enabling various cpu optimizations.
 OPTION(CPUAMD_k8  "Enable AMD K8 optimizations (Athlon through athlon64)" 
     OFF )
@@ -650,6 +649,36 @@ ELSE("${CMAKE_BUILD_TYPE}" STREQUAL "Profiler")
 ENDIF("${CMAKE_BUILD_TYPE}" STREQUAL "Profiler")
 
 
+# On some Ubuntu versions and derivatives, a bug exists whereby enabling
+# PIE compilation (Position Independent Executables) results in the
+# `file` utility incorrectly recognising the compiled vegastrike binary
+# as a shared library instead of a position independent shared executable
+# object.
+#
+# To avoid this scenario, turn off this flag by default and let packagers
+# on other distributions turn this on if their OS is able to correctly deal
+# with Position Independent Executables.
+#
+# For more info, see:
+#
+# - https://bugs.launchpad.net/ubuntu/+source/file/+bug/1747711
+#
+# - https://github.com/vegastrike/Vega-Strike-Engine-Source/issues/94
+#
+UNSET(COMPILE_WITH_FPIE CACHE)
+UNSET(CMAKE_POSITION_INDEPENDENT_CODE)
+OPTION(COMPILE_WITH_FPIE
+  "Enable Position Independent Executables (NOT RECOMMENDED on Debian/Ubuntu/Mint)"
+  OFF)
+IF(COMPILE_WITH_FPIE)
+  message("!! Enabling Position Independent Executables (NOT RECOMMENDED on Debian/Ubuntu/Mint) !!")
+  SET(CMAKE_POSITION_INDEPENDENT_CODE ON)
+ELSE(COMPILE_WITH_FPIE)
+  message("++ Disabling Position Independent Executables (Recommended on Debian/Ubuntu/Mint)")
+  add_compile_options(-fno-PIE)
+ENDIF(COMPILE_WITH_FPIE)
+
+
 # Turn off compiling vegastrike bin
 OPTION(DISABLE_CLIENT "Disable building the vegastrike bin"
 		OFF )