C: Make C bindings always on and embedded into libadios2

CMakeLists.txt

bindings/C/CMakeLists.txt

bindings/C/adios2/adios2_c_enums.h

bindings/C/adios2_c.cpp

bindings/C/adios2_c.h

bindings/CMakeLists.txt

cmake/DetectOptions.cmake

source/adios2/ADIOSConfig.h.in

source/adios2/ADIOSMPI.h

source/adios2/ADIOSMPICommOnly.h

source/adios2/mpidummy.cpp

source/adios2/mpidummy.h

testing/adios2/bindings/C/CMakeLists.txt

testing/adios2/bindings/C/SmallTestData_c.h

testing/adios2/bindings/C/TestBPWriteTypes.c

testing/adios2/bindings/C/TestBPWriteTypes_nompi.c

testing/adios2/bindings/CMakeLists.txt