properly document FPM_CXX, FPM_CXXFLAGS and options

Author: semarie

README.md

@@ -182,11 +182,13 @@ The table below lists the environment variables that control `fpm`'s choice of c
 compiler flags, archiver locations, and link flags, each of which can be overridden by 
 passing `fpm` flags also shown in the table.
 
-| Environment Variable | Defines               | Overridden by  |
-| :------------------- | :-------------------- | :------------- |
-| `FPM_FC`             | Fortran compiler path | `--compiler`   |
-| `FPM_CC`             | C compiler path       | `--c-compiler` |
-| `FPM_FFLAGS`         | Fortran compiler flags| `--flag`       |
-| `FPM_CFLAGS`         | C compiler flags      | `--c-flag`     |
-| `FPM_AR`             | Archiver path         | `--archiver`   |
-| `FPM_LDFLAGS`        | Link flags            | `--link-flag`  |
+| Environment Variable | Defines               | Overridden by    |
+| :------------------- | :-------------------- | :--------------- |
+| `FPM_FC`             | Fortran compiler path | `--compiler`     |
+| `FPM_CC`             | C compiler path       | `--c-compiler`   |
+| `FPM_CXX`            | C++ compiler path     | `--cxx-compiler` |
+| `FPM_FFLAGS`         | Fortran compiler flags| `--flag`         |
+| `FPM_CFLAGS`         | C compiler flags      | `--c-flag`       |
+| `FPM_CXXFLAGS`       | C++ compiler flags    | `--cxx-flag`     |
+| `FPM_AR`             | Archiver path         | `--archiver`     |
+| `FPM_LDFLAGS`        | Link flags            | `--link-flag`    |

src/fpm_command_line.f90

@@ -148,6 +148,8 @@ module fpm_command_line
     '                    unless set by the environment variable FPM_FC.              ',&
     ' --c-compiler NAME  Specify the C compiler name. Automatically determined by    ',&
     '                    default unless set by the environment variable FPM_CC.      ',&
+    ' --cxx-compiler NAME  Specify the C++ compiler name. Automatically determined by',&
+    '                    default unless set by the environment variable FPM_CXX.     ',&
     ' --archiver NAME    Specify the archiver name. Automatically determined by      ',&
     '                    default unless set by the environment variable FPM_AR.      '&
     ]
@@ -162,6 +164,9 @@ module fpm_command_line
     ' --c-flag CFLAGS   selects compile arguments specific for C source in the build.',&
     '                   The default value is set by the FPM_CFLAGS environment       ',&
     '                   variable.                                                    ',&
+    ' --cxx-flag CFLAGS selects compile arguments specific for C++ source in the     ',&
+    '                   build. The default value is set by the FPM_CXXFLAGS          ',&
+    '                   environment variable.                                        ',&
     ' --link-flag LDFLAGS  select arguments passed to the linker for the build. The  ',&
     '                   default value is set by the FPM_LDFLAGS environment variable.'&
     ]
@@ -181,6 +186,12 @@ module fpm_command_line
     ' FPM_CFLAGS        sets the arguments for the C compiler', &
     '                   will be overwritten by --c-flag command line option', &
     '', &
+    ' FPM_CXX           sets the path to the C++ compiler used for the build,', &
+    '                   will be overwritten by --cxx-compiler command line option', &
+    '', &
+    ' FPM_CXXFLAGS      sets the arguments for the C++ compiler', &
+    '                   will be overwritten by --cxx-flag command line option', &
+    '', &
     ' FPM_AR            sets the path to the archiver used for the build,', &
     '                   will be overwritten by --archiver command line option', &
     '', &