ENH: Use CMAKE_INTERPROCEDURAL_OPTIMIZATION:BOOL=ON

The use of CMAKE_INTERPROCEDURAL_OPTIMIZATION:BOOL=ON allows the linker
and the compiler to communicate more closely.

This will cause compilation warnings if any symbol violates
the one-definition-rule (i.e. a symbol is defined in more
than one translation unit with different definitions).

NOTE:  LTO is not supported with clang on linux with the ldgold linker

.travis.yml

@@ -20,7 +20,7 @@ matrix:
       env:
         - CFLAGS="-Wall -Wextra -m64"
         - CXXFLAGS="-Wall -Wextra -m64"
-        - CMAKE_EXTRA="-DGDCM_USE_SYSTEM_OPENSSL:BOOL=ON -DGDCM_WRAP_PYTHON:BOOL=OFF -DGDCM_WRAP_CSHARP:BOOL=OFF -DGDCM_WRAP_JAVA:BOOL=OFF -DGDCM_WRAP_PHP:BOOL=OFF -DGDCM_USE_SYSTEM_EXPAT:BOOL=ON -DGDCM_USE_SYSTEM_JSON:BOOL=OFF -DGDCM_USE_SYSTEM_LIBXML2:BOOL=ON -DGDCM_USE_SYSTEM_OPENJPEG:BOOL=OFF -DGDCM_USE_SYSTEM_POPPLER:BOOL=OFF -DGDCM_USE_SYSTEM_UUID:BOOL=OFF -DGDCM_USE_SYSTEM_ZLIB:BOOL=ON -DGDCM_BUILD_DOCBOOK_MANPAGES:BOOL=OFF"
+        - CMAKE_EXTRA="-DCMAKE_INTERPROCEDURAL_OPTIMIZATION:BOOL=ON -DGDCM_USE_SYSTEM_OPENSSL:BOOL=ON -DGDCM_WRAP_PYTHON:BOOL=OFF -DGDCM_WRAP_CSHARP:BOOL=OFF -DGDCM_WRAP_JAVA:BOOL=OFF -DGDCM_WRAP_PHP:BOOL=OFF -DGDCM_USE_SYSTEM_EXPAT:BOOL=ON -DGDCM_USE_SYSTEM_JSON:BOOL=OFF -DGDCM_USE_SYSTEM_LIBXML2:BOOL=ON -DGDCM_USE_SYSTEM_OPENJPEG:BOOL=OFF -DGDCM_USE_SYSTEM_POPPLER:BOOL=OFF -DGDCM_USE_SYSTEM_UUID:BOOL=OFF -DGDCM_USE_SYSTEM_ZLIB:BOOL=ON -DGDCM_BUILD_DOCBOOK_MANPAGES:BOOL=OFF"
         - B_NAME=system
     - compiler: clang
       os: linux
@@ -35,7 +35,7 @@ matrix:
       env:
         - CFLAGS="-Wall -Wextra" # -m64 -fsanitize=address,undefined
         - CXXFLAGS="-Wall -Wextra" # -m64 -fsanitize=address,undefined
-        - CMAKE_EXTRA="-DGDCM_BUILD_DOCBOOK_MANPAGES:BOOL=OFF"
+        - CMAKE_EXTRA="-DCMAKE_INTERPROCEDURAL_OPTIMIZATION:BOOL=ON -DGDCM_BUILD_DOCBOOK_MANPAGES:BOOL=OFF"
         - B_NAME=fsanitize
 
 before_install: