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.
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