Prefer generic system compilers by default for C, C++, and Fortran

Teach CMake to prefer the system default compiler automatically when no
compiler is specified.  By default use "cc" for C, "CC" for C++, and
"f95" for Fortran.  Load a new Platform/<os>-<lang>.cmake module to
allow each platform to specify for each language its system compiler
name(s) and/or exclude certain names.

Create Platform/(CYGWIN|Darwin|Linux|Windows)-CXX.cmake modules to
specify "c++" as the system C++ compiler name for these platforms.  On
systems that use case-insensitive filesystems exclude C++ compiler names
that are distinguished from C compiler names only by case.

This will change the default compiler selection for existing build
scripts that do not specify a compiler when run on machines with
separate system and GNU compilers both installed in the PATH.  We do not
make this change in default behavior lightly.  However:

(1) If a given build really needs specific compilers one should specify
    them explicitly e.g. by setting CC, CXX, and FC in the environment.

(2) The motivating case is to prefer the system Clang on newer OS X
    systems over the older GNU compilers typically also installed.  On
    such systems the names "cc" and "c++" link to Clang.  This is the
    first platform known to CMake on which "c++" is not a GNU compiler.
    The old behavior selected "gcc" for C and "c++" C++ and therefore
    chooses GNU for C and Clang for C++ by default.  The new behavior
    selects GNU or Clang consistently for both languages on older or
    newer OS X systems, respectively.

(3) Other than the motivating OS X case the conditions under which the
    behavior changes do not tend to exist in default OS installations.
    They typically occur only on non-GNU systems with manually-installed
    GNU compilers.

(4) The consequences of the new behavior are not dire.  At worst the
    project fails to compile with the system compiler when it previously
    worked with the non-system GNU compiler.  Such failure is easy to
    work around (see #1).

In short this change creates a more sensible default behavior everywhere
and fixes poor default behavior on a widely-used platform at the cost of
a modest change in behavior in less-common conditions.

4 files changed, 24 insertions, 0 deletions

diff --git a/Modules/Platform/CYGWIN-CXX.cmake b/Modules/Platform/CYGWIN-CXX.cmake
new file mode 100644
index 0000000..bf37f79
--- /dev/null
+++ b/Modules/Platform/CYGWIN-CXX.cmake
@@ -0,0 +1,7 @@
+if(NOT CMAKE_CXX_COMPILER_NAMES)
+  set(CMAKE_CXX_COMPILER_NAMES c++)
+endif()
+
+# Exclude C++ compilers differing from C compiler only by case
+# because this platform may have a case-insensitive filesystem.
+set(CMAKE_CXX_COMPILER_EXCLUDE CC aCC xlC)
diff --git a/Modules/Platform/Darwin-CXX.cmake b/Modules/Platform/Darwin-CXX.cmake
new file mode 100644
index 0000000..bf37f79
--- /dev/null
+++ b/Modules/Platform/Darwin-CXX.cmake
@@ -0,0 +1,7 @@
+if(NOT CMAKE_CXX_COMPILER_NAMES)
+  set(CMAKE_CXX_COMPILER_NAMES c++)
+endif()
+
+# Exclude C++ compilers differing from C compiler only by case
+# because this platform may have a case-insensitive filesystem.
+set(CMAKE_CXX_COMPILER_EXCLUDE CC aCC xlC)
diff --git a/Modules/Platform/Linux-CXX.cmake b/Modules/Platform/Linux-CXX.cmake
new file mode 100644
index 0000000..b594dae
--- /dev/null
+++ b/Modules/Platform/Linux-CXX.cmake
@@ -0,0 +1,3 @@
+if(NOT CMAKE_CXX_COMPILER_NAMES)
+  set(CMAKE_CXX_COMPILER_NAMES c++)
+endif()
diff --git a/Modules/Platform/Windows-CXX.cmake b/Modules/Platform/Windows-CXX.cmake
new file mode 100644
index 0000000..bf37f79
--- /dev/null
+++ b/Modules/Platform/Windows-CXX.cmake
@@ -0,0 +1,7 @@
+if(NOT CMAKE_CXX_COMPILER_NAMES)
+  set(CMAKE_CXX_COMPILER_NAMES c++)
+endif()
+
+# Exclude C++ compilers differing from C compiler only by case
+# because this platform may have a case-insensitive filesystem.
+set(CMAKE_CXX_COMPILER_EXCLUDE CC aCC xlC)