Test commands executed by unix C compiler.

No functional changes intended.

Tests check that:

• Compiler is obtained from CC.
• cc is used as the default compiler. Currently not true as a Python build time compiler is used as the default.
• Preprocessor is obtained from CC when CPP is unspecified by adding -E.
• Preprocessor is obtained from CPP.
• cpp is used as the default preprocessor. Currently not true as Python build time preprocessor is used as the default.
• Shell word splitting rules are used to split CC.
• Shell word splitting rules are used to split CPP.
• Deprecation warnings are disabled during compilation.
• Preprocessing step includes CPPFLAGS.
• Compilation step includes both CFLAGS and CPPFLAGS, in that order.
• Macros from CFLAGS are defined only once. Currently not true as they are defined twice.
• Flags that would retain macros after preprocessing step are filtered out. Currently only partially true as they aren't filtered out from CPPFLAGS.
• Preprocessing step includes flag that preserves comments.
• Preprocessing step includes current working directory.
• Complete preprocessing command doesn't contain anything unexpected. Currently not true as Python build time CPPFLAGS are included as well.
• Complete build command doesn't contain anything unexpected. Currently not true as Python build time CFLAGS and CPPFLAGS are included as well.